The TH400 is a heavy duty 3 speed (ratios: 2.48, 1.48, 1.00). If you want strength and gas mileage is not terribly important, this is the one for you. It has no torque converter lockup.
There is also an electronic overdrive version of the TH400 called the 4L80E. It is still in use today in 3/4 and 1 ton GM trucks. In order to use this one you'll need an aftermarket trans controller or switch to full manual valve body (must always shift the trans yourself).
The TH700R4 is a lighter duty 4 speed (ratios: 3.06, 1.63, 1.00, 0.70). It offers improved gas mileage through overdrive and lockup torque converter. In stock form it is not a wise choice behind any big block, but it can be beefed up to live behind mild big blocks.
If the truck is just for towing/cruising/goofing off around town, the TH400 is the one you want.