If you're going to tow the 4L80E or the TH400 and a Gear Vendors OD box are the least expensive way to go. Both are less expensive than any manual conversion fit for towing and they will both hold up very well. Do your own homework but I'd say the GV and a rebuilt 4L80E with a TCM and speedo cable drive will ballpark about the same $$.
The 1985 and later Squares had hydraulic clutches. You'll need one for the NV or ZF gearboxes.
If you don't intend to tow much... The NV3500 is the least expensive manual 5 speed... Still pretty expensive...
The 6.2L with stock peak torque of 257 ft-lb is well below the 300 ft-lb max rated torque of the NV3500 that GM used on the 1993+ ST and 1/2 ton T400 CK trucks.
The NV3500 gearing is similar to the TH700R4. The ratio splits are pretty civilized and there's a low enough 1st gear to make it easy to take off. It's a real 5 speed on the highway and there are 4 usable forward gears in town without a gaping chasm between your launch and 2nd gears.
FYI The 88-92 ST trucks did not have NV3500 transmissions. They were a mix of T5 truck trans and HM290 trans. I've seen the various iterations of the HM290 listed on craigslist and elsewhere as an NV3500... they are not.
The 4.3L ST series NV3500 tranny has a longer tailshaft than the 4.3L CK version.
The other manual options are more $$$$ and lots of time...
The NV4500 is a cool truck transmission but it's expensive to swap in. The GM marketing department called it a 5 speed. It's not... It's geared like an SM465 with an additional 0.73:1 OD gear... You can get this same manual gearing setup, cheaper and easier, with a real SM465 and a Gear Vendors OD box... The Gear vendors box won't break up the end of the mainshaft when you tow in OD like the NV4500 5th gear.
The NV4500 gear split from 2nd to 3rd is just like the SM465 1st to 2nd (the SM465 low gear is marked L not 1). This is unpleasant in city driving. 2nd is actually your starting gear making it a 3 speed transmission in town and a 4 speed OD on the highway unless you're towing heavy stuff. I have two of them in 3/4ton trucks. One of them came with it from GM. I wouldn't use one in a 1/2ton and I will not ever do another conversion.
The gear ratios on the ZF S6-650 and the fact that it's tough make it the best of both worlds. But. It's rare and costs a pretty penny if you do find one. It's miserable to install on GM engines that were not made in this century. And. It's darned expensive to repair if you can actually find repair parts.
