Alright, since you had trouble before, lets go over what is needed to swap from a TH400 to a 700r4 right here
First, you will need to modify your driveshaft. A TH400 is shorter than a 700r4, and most use a fixed yolk type output. This means that you'll need to have your driveshaft shortened, and you'll need to get the u-join knuckle for the fixed yolk replaced with a slip shaft for the 700r4. Since you'll have a slip shaft, you can also switch to a single peice driveshaft right away, if you so desire.
Next, you'll need to move, and modify your transmission crossmember. I'm not sure which direction it needs to be moved in, but it does need to be moved. This means new holes drilled in the frame in the correct position to bolt the crossmember down. Second, you'll need to drill a new hole in your transmission crossmember for the transmission to sit in. Most TH400s I've seen have two bolts on the transmission mount, while most 700r4s I've seen just have a single bolt inbetween where the two TH400 bolts used to sit.
Next, you'll need to replace the vaccum modulator with a TV cable. This means plugging off the vaccum line at the back of the intake, then buying the required linkage for your carb to install the TV cable. Everyone makes it out to be a very daunting task, but a TV cable is extremely easy to set. You just bolt it on, hook it up, set it to the highest pressure possible (slide to to the back of the engine towards the firewall) then grab the throttle linkage by hand, and open it to wide open throttle. As you do so, the cable should pull forwards towards the front of the engine. Don't use your gas pedal to do this, as the gas pedal cables can stretch, and not get your carb to full wide open throttle.
Last major thing you'll need to figure out is the lockup wiring. You can rob a carburated car or truck of it's tranny lockup system, or buy one from TCI or something. The converter lockup is essential to the tranny, you'll burn it up if it doens't lock up properly.
Of course, there are always little things that a guy might come across, esspecially if you are installing it into an older truck like a 67-72.
The only reason why your post was modified was becuase you posted a wanted ad in it along with the question, which isn't allowed outside of the classified sections.