The 7.625 was optional on most g bodies starting in 84, the monte carlo had them a lot. The 7.5 was 26 spline, a common cheap upgrade are used axles from the 8.5 from a 442 or GN which are 28 spline. Nice setup you had there with yours. My regal ran 15.0 at 90 with 2.41 gears, slippy th350s (redneck high stall I guess lol) and my slightly less than 8:1 350 with the comp 268.
The 7.5 internals swapped over to 7.625 internals may have been optional, but the 7.5 with 26 spline was standard, and this is the one I was talking about not being able to hold up to much, though some guys have had some luck. I posted all that upgrade info in case someone was interested. Why take a chance on the factory stuff when it's notorious about breaking at some point, whether it's the axles, spider gears, or ring/pinion walk due to housing flex? The steel strength differences and axle size strength differences are accurate to the best of my knowledge, and the 28 spline axles (factory or aftermarket) will fit the 7.625 and 8.5 rears (interchangeable). In all likelyhood, one would not really 'need' the 1541H steel 28 spline axles (pretty much overkill vs the rest of the rear end)--meaning the 26 spline 1541H axles would suffice. The steel is still 35% stronger than 1040 factory stuff, and has a much greater benefit this way vs the 18% increase from going to 28 spline axles. I've done a ton of research on this axle. The upgrades I listed makes the 7.5/7.625 as bulletproof as it can get. If you need more, you'll have to look into getting another rear end altogether.
As a note, the 26 spline only fit the smaller 7.5 2 series carriers, while the 28 spline axles only fit the 7.625 3 series carriers---to the best of my knowledge. The 28 spline axles are thicker, giving 18% increase in strength over the 26 spline axles. A much stronger upgrade option would be the 1541H steel, which is 35% stronger for both 26 and 28 spline axles. A lesser expensive upgrade path would be to find a 7.625 carrier and axles from one of those to put into the axle housing---but even this is inferior in strength to the better steel. Often times I've seen guys uprade to this lesser expensive path, but they put in a 3.73 or 4.10 gear, which kills it due to axle housing flex which wasn't addressed. It often wasn't noticed until slicks were used.
There there must be a crossover year like the 8.5 in 88 going to 30 spline, the 7.625 must have upgraded axles then also. I have swapped many s10/Astro , camaro/ firebird/z28/TA rear guts into regals and montes. Never ran into an axle that didn't work between them. V8 camaro/ firebird has 7.625 as its cast into the pinion gear. The only issue I ran into was earlier housing (78-82) having a different set of pinion bearing.
Yes, you would need a series 3 carrier as well, or at least one that accepts 28 spline axles. Summit sells some that are 26 spline and allow 3.08 and lower (higher number), and vice versa with 28 and lower like 2.41 if I recall correctly. Been a while since I went 7.5 parts shopping.
