Happy TG ALL. Here's my 2 cents...You have a small motor with "big" heads and low compression (as you admit, it's using pieced together parts) I'll bet the biggest contributor is a dip in the torq curve not allowing the converter to drive thru to it's highest stall at the line. I think it's something a camshaft change could fix, might help advancing the current one. Among other 'minor' issues contributing to the problem that have already been discussed.
