What's fuel pressure when the engine is running poorly?

I'm using GM small cap HEI. I have replaced p/u and module with OE GM parts, drive gear looks new.
I have pulled plugs, #8 and #5 are slightly brighter than the rest. No signs of exhaust leaks
I thought the same zz. The first thing I did was replace intake gaskets, and smoke test. No vacuum leaks! I Vacuum gauge reads in the yellow(15) inches? Needle moves minimally.
Fuel pressure reads about 20 PSI-Holley HP spec for TBI.

Last night I readjusted valves (again!). Used 1/4 turn past zero lash instead of 1/2 turn. Engine runs better, but still not quite right. Next step is double check timing ensuring that
actual timing is matching commanded timing.

Thanks everyone for helping me out!

Any ideas on what would have changed with less lifter pre-load? Maybe one of the lifter cups was sticking when further down the lifter bore?

What oil grade are you using and what pressures below 1500rpm?

I was planning to use the BBC factory lifters, but maybe I'll go with the Johnsons. They're just quite a bit up here, landed north of the 49th, if only running a relatively mild cam.

Forgive me for pointing out something you probably already know. Hydraulic lifters can be a little tricky to adjust. By the time you get to the rocker there is a chance the lifter has bled down already. So it can be difficult to tell where the 0 lash actually is. Has always been my preference to do hydraulics while the intake is off so I can see the plunger.

Another thing - I always use a remote starter switch to crank the engine. I don't bother looking for or trying to keep track of TDC. Make sure the engine is not going to start. Key off, kill switch and disconnect fuel pump.
Adjust intake when exhaust starts to open
Adjust exhaust when intake finishes closing

Have you hooked up a vacuum gage. Does the needle bounce. Bounce between what two trains and how fast? You can look up vacuum gage reading online to help you diagnose.
As for pulling plugs, hook up s tack and record rpm drops for each cylinder disconnected.

I'm not sure a smoke test would alert you to an internal vac leak. Fortunately it easy to check. Remove PCV, plug all valve cover holes, rmv oil fill cap and place the palm of your hand over the oil fill hole (best to do on a cold eng). You should feel crankcase pressure start to build at idle. If you feel vac pulling on your hand, you have a leak where the intake meets the head/s but on the bottom (crankcase side). Also if you have a PCV Valve, could try pinching off hose (or pluggng it) and see if it smooths out. Another easy thing to try is partially blocking air coming into throttle body which will enrichen mixture and if it's lean, it should smooth out.

Would be good to compare idle intake manifold vacuum readings between the 2 engs. 15" may be normal for your cam, or may be a little low. Hard to say.