You've got an interesting year there Sand. You're right, its a Turbo 400, and technically, in 90 is when GM changed the naming system of their transmissions, so technically speaking its a 3L80 (3 speed, Longitudinal Install or Front to Back where FWD gets a T for Transverse install, and 80 for the strength or Torque rating of 8000 GVR)
This also when the 700r4 became the 4L60. Then in 93 the 700r4/4L60 became electronic so then became 4L60-E. Had the Turbo 400 ever became electronic, it would have been the 3L80-E, but the Th400/3L80 was replaced in 91 by the 4L80-E, and just to throw in, had the Th350 made it into 90 model vehicles it would have been renamed the 3L60. So there is that preface of info I already knew. But what I was wanting to figure out, and with help from Retro to confirm, I think 91 is also the first year of the electronic speedo, so that is all fine and well since the 4L80-E is equipped. I knew the Th400 in 87-90 gots its kickdown info from the ECM instead of throttle mounted kickdown switch, but I was a bit curious of how the ECM knew to activate it and overide it by VSS. Obviously it does not have a VSS and that is what I was trying to figure out. If there was a Th400/3L80 model will VSS in the tailshaft. Apparently, and I am just making an educated guess, the kickdown is only activated by the ECM from a read off the TBI TPS and that is it. All other function of the trans remain the same, as the governor will override the kickdown relay. Anyway, just thought I'd throw some brain exercise out there. Not that is really applies to you now that you found your answer on your own. You ever get a wild hair, convert that dude to 4L80-E. Doh, wait a minute, unless you are willing to change out the instrument cluster too, and ECM and blah blah blah, probably not feasible. If I get a 4x4 3/4 Burb, I don't think I'd have a problem running 4.10 and Th400 so long as I was running a tall tire cuz its not likley to be my cross country vehicle sucking gas at 8-9 mpg.