By early, I'm referring to an '89-'91 core. I, for sure, know those all utilized mechanically (geared) driven speedometers. I don't recall how far into the '90s they still utilized gear driven speedometers. My memory is telling me that '92 (when the trucks were given a facelift) is when they went to a speed-sensor driven speedometer. That may be flawed.
All of that said, I'm presuming given your earlier statement you're going to build one or have a knowledgeable individual build one. As somebody referenced after I said find an early core (for the gear-driven speedo), the downfall of the early units is there are quite a few weak links in them. I'd have your core built with late(r) model E4OD and 4R100 parts and updates. I'm speaking specifically about the updated/better center support, the 4R100 steel planetaries, updated/stronger sunshells and high capacity clutch packs. I'd have a Transgo Tugger shift kit installed in it when I rebuilt it as well as installing some Sonnax parts (the aluminum pump anti-drainback valve instead of just drilling the pump out to bypass the junk plastic valve Ford installs; and modulator valve modifications).
The Baumann box is pretty straightforward and simple. Their website has schematics of everything required. I don't believe there is a vacuum port on the box. You'd need to mount a throttle position sensor on your carb (or its linkage) to give the box a signal indicating how much skinny pedal you're feeding the rig - which will signal to the controller how much line pressure is needed and the shift point instead of a vacuum signal like on your C6. The cool thing is you can reprogram the line pressures and shift points to your liking - you can set it up, for example, so that if all the TPS/controller is "seeing" is say 25% throttle opening, it'll shift 1-2, 2-3, etc. just like a stock late model truck would as it rolled off the Ford assembly line (read: fairly subtle, smooth shifts). However, if the TPS/controller is seeing let's say 60%+ throttle opening, you can have it set up so that it boosts line pressure and makes the shifts much more firm and quicker. And all of that is adjustable...unlike sticking a shift kit in your old hydraulic C6 which, once the kit is installed it's a you-get-what-you-get shift pattern, firmness and quickness until/unless you get underneath the rig, pull the pan off, drop the VB
, and install some different springs in the VB
The lock-up converter will be your friend, for sure. Depending on how loose the converter in the rig is currently, you could probably knock a couple hundred rpm off of cruising speeds as-is with your C6 if such was available.
As somebody else referenced, if you don't mind going the hand-shaker route, you could probably do what you're wanting to accomplish for less money by going the ZF route (I'd try to find a 5S-47 model out of a later model truck...i.e. '96 or '97...the 5S-42s in the '87s-'95ish models are a weaker unit).
EDIT: don't let the TPS verbiage I spewed about scare you. It's a simple modification/addition to make. And an inexpensive TPS from something like a Fox 5.0 Mustang works perfectly fine and they are plentiful at parts stores and inexpensive. A friend of mine ran a 5.0 TPS on a carbed 306 small block in a Fox Mustang years ago while running a 4R70W/AODE in his Fox Mustang. He/his car actually was something of a test mule for Baumann as he lived right near where Baumann is located in upstate South Carolina in the Greenville area. This was in the mid to late '90s and he was competing in some Mustang drag racing series...running an 4R70W/AODE in a class that, at the time, was predominantly occupied by T-5 jammed cars.