Idle drop by 500 rpm?

Ironracer said
I closed the mix screws, brought em to highest idle (2.5 out) and messed w/ the timing...

2.5 out? should be 1.5 max.
I think you should look for a vacuum leak.
Holley 750's are factory spec'd for 396 Chevy. if anything it would be rich for a mild 340.