This swap is one of the most common ones i've seen around on the
internet, and I figured I'd chip in my two cents worth.
Note: There were two types of 700-R4, the earlier models, and the later models. I'm not talking about the real late model stuff; the 4L80-E, but the strength of the transmission. Pre-87 transmissions were considered weaker than the newer ones. The easiest way to determine what kind of 700 you have, is remove the torque convertor, and look at the pump casting. If the last three numbers are 832, you are in luck; you have the newer transmission with the bigger input shaft, and bigger clutch packs. Although there is nothing WRONG with earlier models, don't expect it to hold up when you're beating the hell out of it.
The problems you run into with these transmissions and a pain free swap
are:
The crossmember transmission mount location (in my car, at least) was about an inch too far ahead. So what I had to do; was drill a new mounting hole; and cut the excess off of the crossmember to clear the larger pan on the 700 as opposed to the short one on the TH-350. Apparently the 700 shares it's mounting position with the turbo 400 transmission. I have no way of verifying or confirming this, so it is merely a stipulation.
To wire up the torque convertor; you will need to order three parts from the General. I have listed below the part numbers
Run a wire from a hot ignition source through the brake switch, to the vacuum switch, to the A terminal on the tranny. Then route it to the 4/3 switch ont he valvebody, and then to the grounded solenoid. This will complete the circuit; and lock the convertor up in overdrive, and unlock it during panic stops, hard acceleraton, and in first, second, and third gears.
On the 700 I bought; it was equipped with an electronic speedometer sender plug. I simply called my GM dealer; and ordered a cable driven one for my particular gear ratio and tire size, and put it in in place of the electronic sender. No other modifications needed.
I personally replaced the throttle valve in my transmission to allow me to mash the pedal 3/4 of the way down before it drops down a gear to accelerate. A point of information to all attempting this swap. If the convertor does not lock; extreme amounts of heat will result, resulting in the death of your transmission. Just a warning.
Remember you will have to modify your shifter detent; or buy a new shifter to accomodate the extra gear position. Three speed positions are PRND21, whereas the 700's gear positions are PRN(D)D21, the extra gear being the over-driven gear. If you don't ever need to put the transmission into low gear; don't worry about modifying your shifter. However, if you want a torquey gear to make a smokeshow to impress people - The shifter must be modified/changed.
I take no responsibility for any errors or omissions on this page.