As far as Transmissions if you want bulletproof to beat on for fun you want a four speed! The 904 should just be rebuilt with good parts $120. Maybe a few things in there to beef it up for a few dollars and like was said at least a t2 if not at t3 transgo shift kit.
Here's the problem with an eight and three-quarter, it costs less or just the same to do a Dana 60. Because they're not so sought after or popular or whatever. And as far as being able to change your rear end gears yes it would be easier but nobody really does it for the most part. Now you will get one or two Knuckleheads on here they say they do but probably not too often. Really the only major easier part about doing that is you don't have to reset the Lash you can just put a different pumpkin in with different gears. Found a Dana 60 with good 4:10 gears that I'm currently using for $160 on Craigslist. I bought $75 end caps from dr. Dif. $300 35 spline axels with green bearings in any size shape or form. the nice part is this is something you can do slowly on a tight budget. For eight and three-quarter it's going to be hundreds and hundreds for anything even just a housing. And in the end you get a weaker rear end. If you want to ask me how I know after I blew up my 489 case 8 and 3/4 rear end that I just rebuilt 20 a quarter mile passes before. They do break or anything can be broken.
