Can the internals of a 7.65 inch rear fit in the 7.5 inch rear? i know the gears do so why cant the posi? Does this make it stronger and how much a setup like this will handle.

 

It is the same thing. Totally interchangeable in every way (except for the 2-series and 3-series issue, and 26-spline axles & side gears vs. 28-spline). I have no clue why the factory "changed" the designation for that model of rear without changing anything about it.

Since they're the same, it does not make it stronger.

28-spline stuff (90 up) is stronger than 26-spline (88 back) in the area of axles. The gears and carriers are the same.

If you want to strengthen your rear, weld the axle tubes all the way around where they join the housing; use a stud girdle type of cover; use a solid pinion bearing spacer instead of a crush sleeve; use synthetic fluid.