You can install either an earlier 904 or 727, but you'll have to change the driveshaft if you go with a 727. If you are going to upgrade the rear axle, this may need to happen, anyhow. Be sure to post your exact engine combo, as you need to deal with engine balance issues, depending on whether a 360LA, 5.9/360 Magnum, etc.
The cable shifters are easy to install, just follow the destructions carefully. Biggest failure guys seem to have is not carefully routing/ anchoring the cable to keep it away from the hot exhaust. Check out B&M or many guys really like the "Cheetah" shifter
Your biggest issue, if not aware, is you'll either have to change the front "K" crossmember, or buy aftermarket "conversion" mounts, as the /6 K member is different for V8s
