904 trans slipping

I have a 1975 Duster and just started looking into getting my 904 rebuilt/doing it myself. The place down the road that specializes in classics said if I bring the car down there they'd take care of it for around $2000, just to give you an idea. I'm in Grand Rapids, MI. Looks like I can get the parts for less than 200 and it's something I may be able to do. Since I have a spare hanging around I might give it a shot. That said, I also like to drive my car. Not everyone who owns a classic is trying to be a mechanic.