high rpm hesitation...need help

[82 GT]

My problem is my engine struggles just before 5000 rpms except in 1st gear, which is fine(for some reason)
It's almost as if I'm running out of fuel. It runs perfectly fine up until that point though.
It only started doing it after I increased my advance in the dizzy. I have no detonation and the car pulls much harder since then, until it hit about 5K in 2nd and 3rd.
I also recently installed a smaller crank pulley to underdrive the accessories and because I was tossing belts left and right.

My theory is that maybe I have maxed out the limits of my intake and carb and that it's choking off the engine at high rpms.
I was also thinking maybe I should back my timing down a few degrees. Can I have too much advance even though I'm not experiencing detonation? My total timing is at 38* and it's all in by 2500rpms.