Looks like gearing to me. I think you need a minimum of a 4.56 gear, maybe a 4.88 in it.
The timing looks fine. Mine runs the best w/ 34-35 degrees in it.
I wouldn't touch the converter until you change the gear.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax