Pretty sure there is 2 different output shafts that were used. Up to 00 they were all 32 spline outputs, but sarting in 01 the 4l80e or 4l85e in a 3500 truck had the MUCH larger 29 spline output shaft. All the actual mounting flanges for the transfer case are supposed to be the same for GM, just makesure it has the 32 spline output shaft in it. As to 4l80e vs 4l85e, there are some improvements in them, but for no more power than a 6.5l puts out, it isn't really needed. The torque rating only goes up a little, but the primary differences are more gears in the planetaries, and I believe the 4l85e had straighter cut gears in them vs the 4l80e. It made them slightly louder, and only slightly stronger. If you can get one from a late 99+, the improvements in them is worth it. And reman torque converters are not that expensive. Last I heard, people were buying the transtar billet cover single disc for well under $500 with core(heard a few have found them for $325, but I can't confirm this personnally), so if it comes from a 6.0l or something else withahigh stall converter, it isn't the end of the world to get another converter.