manual to auto, pickup to burb, 3/4 to 1/2 ton, tranny question!

[speesh]

Hi there,
I have a 71 Burb 1/2 ton that I drive daily, and it now has a 3 speed manual tranny. I want to change it to an automatic, and I happen to have a long wheelbase '69 3/4 ton pickup sitting around with a rebuilt Turbo 400 tranny in it. What would it take to change this over? I know I need a radiator, tranny cooler lines,flywheel, starter, crossmember, and shifter (all of these can come from the '69, right?). My big question is about the driveshaft: actually 2 questions.
A: will the driveshaft from the ' 69 longbed pickup be the right length for a suburban?
B: what about the rear u-joint? are they the same from a 1/2 ton to a 3/4 ton? I doubt it, and if not, what do I do about it?

I tried using search but couldn't really figure out how to ask it what I was searching for!

or should I go for a 700R4? I would like better mileage, but I already have 3.08 rear gears, would a 700R4 still be an improvement? I hear they are not as reliable and I already have the TH400........ I already get about 15 mpg on the highway, is that pretty good and will it go down if I do this changeover?


Dang, I ask a lot of questions!