Just to clarify, I wasn't implying that you should use a '91 80E. Just wanted to make the point that the factory PCM in your '90 might be capable of controlling it if you found the right tuner. Personally I'd go with a '97-'99 4L80E if possible, as they were the most updated of the 80Es while still retaining the old SBC/BBC bellhousing bolt pattern. Anything newer will have the LS bellhousing bolt pattern and won't work with a TBI or Vortec engine.
Like I mentioned above, you need to make sure you don't get one that's too new. Otherwise it won't bolt up to a TBI or Vortec engine in these trucks. '99 or below should be OK.
EDIT - Unless he's LS swapping the truck, in which case he'd want the opposite, 2000 or newer.