If you have already drilled a hole keep drilling the hole until you can get a punch and crush the remaining away from the threads. Plug must have really welded itselg to the manifold. To keep useing easy out it won't work just carefully crush away from the threads.

Turn the 4 or 2 corner adjusting screws out a half or so , sounds like a lean situation.

Put O rings onthe shaft

All good suggestions are waranteed, however you would be most benifited to be sure the engine you have is runing the best it can. That would be correct timing, all vacumn accesories are working properly, dist and carb. Correct jets in carb. I would run a single wire HEI dist simple, work well.
When all that is correct than you can advance to different heads, manifold,exhaust, bore size etc. All the accesories you mention don't amount to more hp only to help in obtaining the above. When the engine is runing the best then what it has to push,turn pull will come into play. EX. disconnect all compentents and turn the engine to any rpm you want, hook it up its a different story. Everything componet on or hooked to the engine is relevent to its performance.

All the things you mention help the engine to run more effeciently, however you need parts to make more HP, such as heads,cam comp ratio etc. Again not being familuar with jet pumps and what HP they demand, I would say as you explain your motor I would say you are closer to 300-325 hp .

I put all my MSD aside and put a HEI on only need one wire works jus great.

Sound like you are not getting into the secondaries. Be sure the vacumn diaphram is moving and not stuck from sitting. Check the spring, if yoo don't already have put the lightest spring in.

Chevy has, had lifter bore issues, take a small brake hone and clean them up. The lifter needs to be able to rotate, even if you find something else it would be preventive maintance to do so.

I have had the same problem and it is a small crack in the manifold. To test plug the exhaust end and put a little air pressure and a guage and you can see.

Be sure the air bleeds are clear, spray some W-D 40 on them, back the corner air screws out a quarter or so.

I am told Finneys

Go at least with a 2 o 3 qt accumulator if you don't change

Also a lot depends on the engine compression the higher the comp ex. 13,14, to 1 the top bearing get squeezed tighter rather than a 8.1 comp. the wear when running a high comp engine the top bearing bears the brunt of wear, like a diesel.
The more the engine is put together the more better oil is necessary.