i might take a different approach.
I'm walking in the blind here so i don't want to offend because you have looked at things on your end and settled on the valve timing being off, which is possible.
1st question: did you ever do a compression test? (just want to know the engine is healthy. You have no idea how many fresh rebuilds i've seen and they didn't do anything on the bottom end and the engine is just beyond worn out.)
2: did you ever remove all vacuum accessories from the engine plug the hoses and run the engine with no vac accessories and see any improvement on a vacuum gauge?
3: test vacuum accessories for leaks? it seems funny but i took a vacuum hand pump and manually inspected my brake booster and A/C system and discovered major problems and leaks that required a new vacuum booster and check valve and major repairs to the A/C system to remove the vacuum loss, bad vacuum motors, bad octopus(vac manifold for AC motors), bad vac motors, just to start.
4) what spark gap are you running? i discovered in my situation going beyond .35 gap caused the engine to be VERY lazy and detonation happy, and had low vac, it acted like retarded timing even running 15 degrees of initial.
5) what type of ignition wires and ignition system are you using? I have discovered major problems with aftermarket wires and some ignition systems that act like a spark delay and completely mess up the readings you get from the timing marks verse the actual timing. i showed my engine builder this and even he found it interesting. depending on your situation i would recommend going to ford OEM wires and installing points during testing to remove electrical abnormalities, after testing is complete then i would go back to electronic and see if problems reoccur. if the engine has any of those spiral core 8mm-10mm ignition wires i would recommend removing them taking them to the engine builder and strangling him with them, then install normal carbon filament OEM wires at least for testing. some electronic ignitions can cause interference and have problems with resistance wires and low voltage that can lead to a retarded electrical timing situation even if the engines mechanical timing is set way above 20 degrees. that can lead you to think the cam timing is all wrong. you get this like delayed spark so you have the mechanic set advanced because the electrical end is retarded and you cannot explain why. its a combination that can occur with electronic ignitions and after market wires. so for a base line i recommend binning everything aftermarket and going OEM to start.
6) how big a carb does the engine have? if a carb is too big for an engine the vacuum will drop because the engine can suck in too much CFM air. The carb will act like it is lean all the time, the engine will show low VAC, causing you to constantly richen the mixture. if an carb is too small you get very high Vacuum at low speed and idle and the carb runs super rich all the time. but when you rev it over 2500 the engine falls on its face.
7) how big is the exhaust? i found on a normal cleveland going over 2.25-2.5" exhaust causes all kinds of problems with low vac and fuel signal issues.
i would want to explore some of these in detail before ripping the front of the engine off to inspect the timing chain and gears.
basically in my situation i took my old worn engine to be rebuilt and i even had it dynoed after rebuild. I expected a turn key engine, and i warned the builder i would need very high vacuum.
what i got was an engine that claimed 304HP on a dyno made 6"HG at idle, Ran like pure garbage, detonated like crazy, and an engine builder that told me to stuck a thumb up my butt for his shoddy work.
It took me forever but without modifying the engine internally i was able to make changes to fuel and ignition and get 18"HG at idle from the same engine. granted my engine is still a bastard when it is cold but after 10 minutes it works as it should. with my engine parts combo things will never be 100% right but like you i was fed up with making more changes.
you might be amazed at how something trivial might make a 4"HG increase in the vacuum of a engine.
now i have seen friends cars making 10-12HG at idle and they don't have issues with the A/C vacuum requirement or the brake booster requirement.
another thing is how the vacuum lines are routed. for example i diagnosed a situation where applying the brakes shutdown the Vacuum motors of the A/C system
this turned out to be a 2 fold issue.
the first thing was both the vacuum booster and the A/C line to the vacuum canister shared the same vacuum stem off the intake manifold. this caused a race situation were both major Vacuum systems were competing for vacuum from the same stem, there simply was not enough capacity in that area to allow both to work. i could of also moved one of the vaccum lines to the front of the intake manifold this would of balanced the load front to back on the intake manifold. but the carb in this cause had a special vacuum plate for the booster that was before the ventri signal so i made sense to just do that and not run a new hose.
the brake booster was then changed to the carb base and the A/C vacuum stem left in the same spot on the intake manifold the issue was reduced 80%
the second problem discovered was the vacuum canister had a blown check valve internally. The vacuum can is suppose to hold vacuum well after the engine is shutdown
just like the vacuum booster is suppose to hold vacuum for at least 2 brake pedal presses. so while there was no leak in the vacuum system the engine had to constantly feed the A/C motors there was no build up of capacity to help the engine, so when you floored it the A/C system shutdown again. i took the vacuum canister out and manually inspected it, i used a hose blew air into the canister inlet. that was when i discovered the check valve was blown in the canister, replacing the canister then solved all the issues. and the engine made 1HG" more vacuum at idle because it was not constantly feeding the A/C system.
there could be more issues, as well with the A/C system. the evaporator has a sensor on it to make sure the system doesn't freeze and it kicks the compressor off when a certain temp is reached internally. the expansion valve could be getting stuck and shutting things down as well. but basically Low vacuum from the motor would effect the vacuum motors for the AC system to blow the air through the vents. the compressor is constantly running off the engine, so unless kicking the compressor on is just stalling the engine out that is a different story. If the system runs cool and over time 5-10 minutes stops cooling then kicks on i would suspect the temperature probe for the evaporator is wonky or the expansion valve is not working and locking up the system. in your situation you said problems with the brakes as well, so that low vacuum would go back to other things i discussed above.
you could end up in the situation i had it wasn't just one thing it was 30 Small things adding up to a big problem, you have to press through one at a time and it drives you nuts
Well sorry for the wall of text maybe food for thought?
