Ok, a few things.
Yes, there is a VSS on some trucks in 1986 at the transmission or transfer case end of the speedometer cable - this was used back then for the automatic speed control and it is the same unit used in the 89-91 trucks that were factory equipped with E4ODs. After 1992, the rear ABS sensor on the differential was used to supply signals to the speedometer, odometer, speed control AND ABS.
Drive shafts will have to be changed, since the E4OD is much longer than a C6 (remember, an E4OD is internally a C6 with an overdrive added at the input and a much more complex valve body which is electronically controlled).
Shift linkage is close enough to work.
However, the lever that actually connects to the arm on the side of the transmission will have to be either lengthened or replaced - again, due to the fact that the E4OD is longer.
The cross member could probably be reused, but I just took the one from the donor truck. In either case, the transmission mount needs to be moved back.
Cost was not be significantly lower than buying a GV overdrive in my case, but if I were to take out the few mistakes, it could probably be done for $2000. Keep in mind, I did not merely swap in a used transmission of unknown history, instead I bought a used one, rebuilt it, (then had to rebuild it again since it had the wrong bell housing
) then put it in the truck. It took time again after that to get it working right. That included the cost of a new billet torque converter and new baumaun TCS.
In my case, I wanted as low a cruising RPM as possible, so although a GV overdrive offers 6 theoretical forward gears, the final drive ratio of a C6 with auxiliary overdrive will never compare to the top gear ratio of an E4OD.
Not only is the GV overdrive ratio less than the E4OD, but the C6 does not have a locking torque converter, so that slip will add on top.
I personally wouldn't bother with the OEM control setup, since there are more things to go wrong with them. I've had VERY minor issues with my baumaun TCS over many years. Even when something did go wrong, it was never expensive or time consuming to fix. Tech support was also excellent (but also rarely needed).