If your getting blow by on the pistons, I would go ahead and pull the motor. Then pull the heads, intake, accessories, pulleys, water pump, timing cover, oil pan. Then take the rest to the machine shop. Have them go thru it and check it all out. It may be fine, and just need a re-ring, it may need some work, but at least you will know and wont be guessing and setting yourself up for failure, engine failure that is. Adding alot of extra power to a worn out engine is asking for big time trouble. So its better to go ahead and bite the bullet and do it right the 1st time instead of picking up engine pieces out of the road.
So, find a good machine shop. Take them the loaded short block. They will check out all the bearings, pistons, cylinder bores, crankshaft, crank bores and machine accordingly. I might suggest if you have to re-ring it, to go with some nice pistons that up the compression a TINY bit, not alot or you will have to use Premium fuel exclusively and no one wants that, unless you do on purpose. Then you could consider a cam swap, just work that out with the machine shop, tell them what you want to achieve, and let them pick the head/intake/cam package. The Edelbrock top end package is a proven time tested combo that will suite 90% of everyones needs. If you want more than that can offer you can go for some trick flow twisted wedge cnc ported heads or some AFR cnc ported heads. Just talk it over with the machine shop so they can do what is needed to make the ENTIRE package perform like you want it to. Whatever heads you choose they will likely have to re-deck the block and possibly tweak the new heads as well. Also your pushrod length will almost certainly change. You will also want to use some nicer roller rocker arms with those new heads. Your old cam and lifters wont be worth re-using either. You will likely need a new carb to go with this, and a new fuel pump and filter. I would also discuss the merits of some low cost additional displacement using a stroker kit. You can build it as a 347 stroker relatively inexpensively. Theres no replacement for displacement, but if you dont need it, its money saved.
As always remember the domino effect. Nothing corrupts like power. Taking a motor that in 1971 had 210hp and now 46 years later has somewhat less than that remaining, then doubling it can have a deleterious effect on any weak link down stream. Putting a motor in that has 350-400hp to replace one that had 210hp can cause many issues, such as exploding the "who knows how long ago it was rebuilt if ever" transmission, or the probably never rebuilt 8" rear end. At the bare minimum you will have some U joint failure after standing on it a few times and you will be picking up the drive shaft out of the road and calling a tow truck.
If your transmission is weak, consider upgrading to a C6 or AOD.
And as always dont forget about STOPPING. If your using stock 4 wheel drum brakes that maybe were looked at 10 years ago, you can go ahead and factor in a disc brake conversion, at least on the front. And if your thinking that your stock 14" drum brake rims will work with that, think again. Most of the kits I have seen such as the Wilwood conversion for 14" wheels state that they will NOT work with 14" wheels from a front drum car, but they do provide all the clearance measurements so you can measure and judge for yourself. If you only have manual brakes you will want to do a Brake Booster conversion to go with it and a new Proportional valve. If your keeping the rear drum brakes, and theres nothing wrong with that, I would at least change the wheel cylinders, shoes, drums and all the hardware. I would also flush all the lines and seriously consider replacing all the lines with the stainless steel pre-bent line kits that CJP sells.
So theres a little more to it than just poping the old heads off and dropping the new ones in place.
