Spoke with Silver Star Transmissions.

Very knowledgeable fellows and advise that you cannot use any of the later 722.3 transmission ('91 or '92) with first gear start. There is a misalignment with the case and the unit won't last 100 miles, although it will bolt in just fine.

The recommendation is to find a valve body from a 380 (722.310 trans) which had first gear start and matches perfectly with the existing trans.