I just started this same repair this week, although mine was broken when I bought it, so I'm not sure how the damage occurred. Mine's also in a 1970 El Camino, so I'm not sure how much room you have, but I was able to get everything apart with the transmission still in the car. I think most of the guys would tell you to take it out, but I don't have a lift. Anyway, it's pretty straightforward, just make sure you have a good pair of lock ring pliers and a 3-jaw gear puller.
BTW - I had to buy a new gear, synchro, blocker ring, and snap ring set and it set me back $340 from D&D Performance.
Ryan