I'd sell your 8.75 and build a Dana 60 starting w/ a junkyard truck rear. My dad did exactly that w/ his 440-6 4 speed '69 road runner. After selling his 3.55 suregrip 8.75, he was only a couple hundred bucks short of breaking even on a 35 spline, 3.54 gear Powerlock diff. Dana 60.

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