Need advice on transmission upgrade

[drl]

I have 1988 V30 Chev crewcab, with 6.2 N/A, 3 speed Turbo Hydramatic 400, and NP205 Transfer Case, 148000 miles. Well the transfer case is leaking and a tooth off the starter ring gear has gone missing so some stuff has got to come out. I want to upgrade the engine/transmission for better fuel efficiency as we want to put a camper on it and travel more. It is a real nice heavy duty truck. Was wondering if the TH700R4 4 speed O/D transmission. with locking torque converter, would fit in exactly to replace my 3speed. Hoping to get to the OD without having to modify the aft transmission mounts and mess with the drivelines. Otherwise I may consider the Gear Vendors Overdrive as option. The locking torque converter must have some desired advantages also. Appreciate your thoughts on this subject. More questions later.

[mrwilecoyote]

If you wanna stay with an auto, use a 4L80e with an Optishift controller or Compushift.
I personally don;t like TH700's although they can be beefed up some it's no comparison to a 4l80.
The easiest thing to do would be put an NV4500 in. Then you got yourself bullet proof and simple.
Another alternative would be a Gear Vendors overdrive and keep your TH400 but again...pricey..
As far as power, A turbo and either turn the pump up or do what i did and swap a 6.5 DB24911 IP and turn that up a little but a turned up DB2-2829(what you have) is usually fine. Nice big 4 inch exhaust.
A 6.5 Turbo setup is cheaper and works the best but a Banks setup is much easier to install and a close second.
I have done the NV4500/Turbo swap to my 86. At some point soon, I will re document it here.