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What are if any options to upgrade the stock rear end, I do have a posi and 373 gears but for a 7.5 diff and I think mine has that 6.5 diff

Biggest upgrade is to go with a 7.5" rear. May require some welding as most H body 7.5" rears used the torque arm and panhard bar along with the two lower control arms. There may be some 4 link 7.5" rears but probably harder to find. I don't know of any upgrades for the 6.5"

Thanks Clyde

Monza 7.5 and weld on the 2 upper control arm brackets.
S10 2wd 7.5 or 8.5 with all the Vega brackets welded on.
Autoweld chassis builds a bolt in 9” Ford housing.

Are your 3.73 gears from a Monza or a different GM car/truck ?
Monza 7.5” housings use a smaller pinion bearing.
Monza pinion gears have a smaller shaft diameter due to the smaller bearing used.
Other GM 7.5” pinions can be machined down to work in a Monza housings.