I put a 700R4 in my 79 this fall. It is a HUGE hit with me! It did indeed make it a different truck. Just HUMS down the road.
I had to drill 4 holes. 3 in the frame and 1 in the x-member for the tranny to mount. If you don't want to mess with elec speedo get a 87 or under tranny. I have a 90 tranny and I am going to have to use a elec speedo or remove the tail shaft and remove all of the speed pick up peices and replace with manual cable speedo. Still havn't descided. GPS works for now LOL!!
Make sure to get a good name brand lock up kit. Bowtie overdrives, painless etc... I have the B.O. kit and it works great, took a hole 20 minutes to wire it from the tranny to the cab and under dash.
I used a B&M trans pack kit in the tranny too. (Recomended!!!) It shifts NICE! and if you plan to even haul a 4 wheeler in the back use a Shift reprogramer kit of your choice. I did my kit on a saturday morning, just find a clean place to do it and a good drill.
Its worth the hastle for a 700R4!