When I was looking for 700r4 for my 78 C20, The trans places I went to asked what I wanted it to do, 400hp, 5200lbs truck tows 8k trailer from time to time, and I dont baby it (unless towing).
They said I need something like a stage 5 or so trans, all the goodies, just not the violent shift.
Also, in a C20 to not have to move the crossmember, to use a 700r4 you'll have to have the tail housing with the mount built into it, its off a 90's caprice police car, dont remember the PN. Otherwise you have to move the crossmember forward, and that will start moving it where the frame curves upward, which wont work.
To put in a 4l80e, the crossmember has to be moves backwards, which isnt a problem, but it may have clearance issues (I've heard) with low hump cab floors.
Here's a site with all the measurements. Look at the "C" measurements.
http://transmissiontechnologies.com/...surements.aspx
As far as the driveshaft, a local shop told me to go from a TH400 to a 700r4, to shorten the driveshaft and install a new front yoke was around $50. So the driveshaft part is pretty cheap and easy, relatively.