personally i would replace the fuel lines rather then bother attempting to flush them.
the various methods to flushing a fuel line:
100% alcohol and injecting it through the line
running fuel through the line with a fuel pump or injecting it through the line.
blowing compressed air through the line.
problem: fuel lines rot from the inside out due to water and the fact the lines are untreated mild steel tubing.
so you flush the line and that gets the loose junk out and maybe some of the flakes but the line will continue to bleed rust particles into the fuel system and eventually you will need to replace it anyway.
if the lines are original they are 40+ years old now.
problem 2: changing the fuel tank opens up a can of worms but its a can that must be opened.
when you drop the tank for replacement you will usually find the Vapor return system (located on top of the tank) is completely gummed up with varnish and deteriorated as well.
That usually means you will be replacing the vapor return line that goes from the top of the tank through the transmission tunnel to the fire wall and terminates going into the blue charcoal canister on the passenger side engine bay aprons or passenger shock tower in 1973.
The vapor line can be replaced with the engine and transmission left in place but it is very frustrating and difficult as it will be a horrible game of tetris. the car will need to be jacked very high up in the air and you will need to start feeding the new vapor return line from the engine bay back to the fuel tank(obviously the fuel tank will be removed, and you might need to pull the parking brake line to help things)
the fuel line by comparison is VERY easy to replace.
now usually with the tank down you would also replace the fuel sender, the newest ones can be bought in stainless steel. 2 problems with replacement fuel senders.
1) the calibration of the fuel level is always wrong and it takes a huge effort to make the float sit correctly, it means installing the sender filling the tank with 5 gallons of gas, and checking the fuel gauge on the dash to see if it is at 1/4, if not you must drain the tank, pull the sender and bend the float arm, then reinstall and fill with gas again. it is a nightmare. optional is if your sender works you could reuse it even if it is rusty just change the sock filter on it.
2) the repop fuel senders had a mistake in assembly that caused 2 issues. 1) the fuel output tube was pointed the wrong way(they seem to have corrected this now) 2) it leaks from the connector rubber seal. and if it leaks you have to replace it.... i replaced a new sender 4 times before i got one that was made correctly with tight rivets and a good seal around the connector.
if my original sender wasn't as horrible as it was i would of reused it.
the tank: you will find there are 2 oring seals one for the fuel sender the other is for the vapor return on the top of the fuel tank. they Do not make the vapor return hat on the tank, so clean it up as best you can and transfer it from the oem tank on to the repop tank.
they also make 2 different fuel tanks repop. 1 is the standard tank, the other is called deluxe, the deluxe tank features a galvanized coating inside and out that is a good rust protector.
the standard replacement tank will start to rust immediately on the outside.
fuel lines and vapor line types
they make replacement mild steel lines and they make stainless steel lines. both have pluses and minuses.
the steel lines are cheaper and very easy to bend and install, but they rust and if you make a mistake during install you can Kink the lines and you will have to trash them and start over.
the stainless steel lines are expensive, VERY difficult to bend and difficult to install it is like dealing with a piece of spring steel, they are very hard to kink, and they will never rust again for the life of the car. the stainless is also so hardened that any road debris will shrug off a direct hit. a standard steel line can crush, kink or puncture. it can happen you run over something in the road and it wrecks the fuel line. but this is VERY rare.
I can give my personal option here; use the deluxe tank, use your original sender if you can, but replace the sock, and keep a new stainless steel sender handy just in case. make sure you get 2 o-ring seals with the tank, clean and reuse the vapor return vent on the top of the original fuel tank. If you happen to have a low fuel warning light on your car you must reuse the original sender no matter what, that type of sender is not reproduced and is very rare. for the lines i would recommend going stainless if you can but be prepared for more of a nightmare during install. the fuel line is a 2 piece again very easy install you will need to unfold the new lines as they come bent into a pretzel for shipping reasons. there will be a spring on the tube this is very important you move the spring into the area you need to unbend and this keeps you from kinking the new fuel lines when you pull the old lines do not throw anything out, you will need to unbend the new lines and compare them to OEM to match them as much as possible before install, same with the vapor line if you are replacing that as well.
basically its not rocket science, it takes a bit of effort but all the lines can be replaced. The fuel tank straps are usually a problem as well because the new fuel tanks are slightly different from the OEM tanks so you may fight the straps for a while, i recommend this as a 2 person job. it can be done alone(i did it by myself but it took me a few days)
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Filters.
So our cars come with a filter located inside the fuel tank on the fuel sender. it is called a sock and is made of hard plastic and nylon filter material and kinda looks like a sock that fits over the end.
many times the original sock is clogged up or the netting material has ripped allowing contaminates into the fuel system.
many carburetors have internal brass filters in the fuel inlet to the fuel bowls. people like to add another filter after or before the fuel pump to catch anything the sock missed before it gets to the carb.
usually you have a ton of filters on the car if you have a deteriorated fuel system. i have 2 on my car. the sock and i have a inline filter just after the fuel pump. I had to remove the brass filters in my holley carb because they interfered with the AN fuel line fittings i used on my Double pumper.
the filter is important because it catches rust but anything else that can get into the tank. you have no idea the amount of junk that gets into a underground fuel tank at the local gas station. the brass filters usually in the carb are only good for really big chunks so you want a fine particle filter to augment the sock in the tank in case that fails.
DO not use glass or see though fuel filters, these are dangerous and can crack or explode with engine heat.
most enclosed filters are also directional so make sure you connect them according to the instructions, some people install a filter the wrong way then suffer from low fuel pressure and can't find the problem.
usually the filter has a flow arrow on it.
well i hope i didn't make your head explode. basically your replacing the fuel tank at this point so don't do half a job it will come back to haunt you later. do it right the first time.
again there is nothing wrong with normal mild steel lines if you want to save money and some frustration you got 40 years out of the last mild steel lines. ( i went stainless on my car)
