With the 3.73 the converter doesn't have enough track distance to get coupled up. Similar to running 1/8 mile. I'm surprised you run an 8" I would think that combo would need a 9.5" 350 rpm drop isn't much.
Doug  My converter guy didn't even consider an 8" for my combo...similar to yours.
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
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