My guesses –
Because it’s a Q-Jet my first guess would be to check the plug in the bottom of the fuel bowl for leaks. This will certainly contribute to fouled plugs. Most people epoxy them in place during a rebuilt, and one outfit has a screw-in solution (I haven’t read it yet but I’ve read it’s on this website):
http://www.cliffshighperformance.com/.
Any of the following can contribute to fouled plugs and low manifold vacuum - Intake manifold end seals that are not sealing properly, intake side gaskets, carb to manifold gaskets, leaking vacuum hoses, or even a leaking vacuum advance canister.
As noted above, the timing chain could be shot or jumped a tooth - or – if the timing cover/timing tag is not original, it could be in the wrong location. Some aftermarket covers with the bolt on timing tab do not correctly orient the mark. You can somewhat check the condition of the chain by watching the timing mark with a timing light while you raise and lower the rpm. It should move somewhat smoothly up and down. If it jumps all over it could be the slop in the chain is waving throwing the timing off.
The vacuum advance could be dead. Easy to check with a vacuum pump or timing light.
The accelerator pump in the carb could be dead or connected on the incorrect link setting. Easy to check by looking down the carb throats and operating the throttle linkage.
And then there’s the option of checking vacuum gauge diagnostics, which may reveal bad valves, springs, etc. Good luck and let us know what you find.