I lean toward cam timing being the issue. I have done that before, not hard with the multiple keyway setups, esp if you have them on both the crank and cam gear. Another reason to ALWAYS degree your cam. Harder with the head on, but can be done.

That said, I'm going to throw this out there. Is your vac adv set up for timed vac and you're using the untimed port (or vise versa)? That will cause some erratic behavior and in combination with one or more of the carb adjustments being out of spec, it could cause this headache. Of course, assuming your advance mechanism integrity is where it needs to be in the 1st place.

Another though, more of a shot in the dark, but is your PCV valve functioning properly? I once new a guy that had the bright idea to "hollow out" his pcv valve (this guy put a Poncho 301 and metric 200 trans in his 68 Firebird). With his "enhanced" PCV valve, it would lean out and fall flat before 5000. The compression was super low in that thing, like 70lbs cranking pressure and within a couple of numbers cylinder to cylinder. The engine was reportedly from an early 80's turbo Trans Am, so the compression makes sense and I imagine the cam was pretty weak too. Not apples to apples, but an easy check to make.