converting an 8 3/4 to the correct sure-grip

I would agree with your buddy in that you could just hang the entire rearend in the car and hook it up as is, then run it to make sure that it is serviceable. Then if I were you, I would start looking for an extra center case with the desired gear ratio in it to have on the bench. Doing a bench rebuild on an extra center will allow you to get the bugs worked out of your housing install, and if there is nothing wrong with the rear you put in, will still allow you to enjoy the car while you rebuild your extra center. Then, make the extra center what you want, suregrip and desired gear ratio. When it's rebuilt, just swap out the center sections, which should take just a couple hours and you're done. Then you still have an extra case to rebuild to something different should you decide to upgrade from highway gears to something more steep.

IMO, either suregrip unit will work for your application, and if you were to buy a brand new Auburn (cone) style suregrip, it would give you years of service in a highway application. Good luck with your project, Geof