The Thm400 differs in size and some mechanical / electrical functions from the thm700 that came with the truck, mine came with a NP207 transfer case that would not fit the 400 unless you found an adapter plate which I did not. I found that if you want to mantain the 4wd you must consider in relocating the oil filter from the engine because it gets in the way of the front drive shaft. The 400 uses vacumm for upshifting and downshifting, while the 700 uses a TV cable attached to the TBI unit or carburator The 400 uses a electric solenoid conected to a switch(kickdown switch) on the gas pedal or the TBI. I also modified the rear drive shaft lenght to fit the 400.
The old S10 uses a speedometer cable for the speedo and the 400 came wiht an electronic sending unit. I decided to replace the gauge cluster with a newer S10 dash ( the 1988s and newer S10s came with an electronic speedometer). You'll need the wiring and the Digital Ratio Adapter Controler (DRAC). This is a little solid state device that is located near the ecm on the newer models and is used to convert AC current coming from the transmision sensor to DC current to the speedo gauge. Not very difficult with the electric diagrams but almost impossible with out them.
You wouldn't have any trouble with the speedo if you have a 1988 or newer S10