I have a 700R4, I love it and it holds up to all the abuse I put it through with my little 400hp blown SBC, 373 gears and slicks. This is how I installed it:
Bought a conversion cross member, TV cable, TV cable mount, TV cable to Holley carb adapter, transgo shift kit, retrofit mechanical speedo sender, mechanical speedo gear, B&M converter lockup controller, a $75 dollar 700R4 from craigslist that "worked when removed," and had driveshaft shortened to fit.
First test: drive, 1st gear, 2nd gear, neutral, neutral. drive it with two gears for several weeks. pretend this is ok, tell the wife nothing.
Buy 1991 suburban with massive frame and body damage with plan to use transmission and sell the rest. harvest and install transmission and sell everything else (pristine interior and solid engine) for $700 to break even, for now (foreshadowing).
Drive car with all 4 gears for several weeks. Notice trans slip going up hill in 4th gear 4 hours into 5 hour road trip. Manually downshift to third on hills to preserve transmission. 700R4 identifies as a powerglide within an hour.
Take car to transmission shop and throw handfulls of money for techs to make the problems to go away.
3 days and $1600 dollars later I have a healthy, built 700R4.
I would recommend this if you really like working on your car and lying to your wife.
About to spend another $800 to replace the converter because the stock lockup cant handle WOT.