Many excellent suggestions have already been made, and I will not attempt to re-invent the wheel. I had a similar situation with my '85 Silverado. After a complete engine swap with a fresh engine, the truck would not start at all. I tinkered with the timing and then the idle mixture to get it to run, but my performance was terrible - lackluster power, shuddering, unburned fuel smell in the exhaust. One day, sufficiently tired of this, I began spraying carb cleaner around the top of the engine. Aiming at the back of the carb seemed to make the engine speed up. Upon removing the carb, I discovered that I had cracked the throttle plate by over-tightening the brake booster line. Replacing the throttle plate, re-timing, re-setting idle mixture, and now it drives like I had hoped for.
In my situation, I had retarded the timing and enriched the idle mixture far from what would normally be correct, just to get the truck to run at all. Because it ran, I thought my adjustments had helped, when in reality they were just masking the real problem - a massive vacuum leak from my own carelessness.
I am not necessarily suggesting that you have a vacuum leak. You have already checked for that. My point is that sometimes the erratic problems are caused by something that is not on our regular checklist. By your statements, you have had most of your engine apart at some point in this endeavor. Go back to the beginning. Check everything from step one, even if it seems preposterous. Persist and you will find the problem. Good luck !
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