Wait another month or so and try again. I've seen 3+ tenths and 4 mph change from good air to bad summer air.
My car wasn't affected at all by the switch to a Dana from an 8.75...except it doesn't eat ring and pinions anymore. I made the switch back when it was a mid 10 sec 3700 lb car FWIW.
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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