383 rebuild - kinda whimpy on the dyno?

Again, double check the 90deg indexing of all the crankpins, don't just trust/expect that they are on.
If the 7/8 throw is retarded, the timing will be too advanced compared to the other 6 cyls., it will hamper output by either causing detonation in 7/8 &/or short-changing optimum advance on the other 6. If rod bearings getting pounded out w/o any evidence of oil supply issues is a repeat performance, check check check. It's gotta be poor crush or detonation............