..... I think a mild modification to a T-5 tranny with late model rear end would be exceptable to the original guys? I won't tell if you don't

I'm glad you decided to keep it original. Mine was not a good candidate for that....sorry to say the rearend out of the panel is gone.
EDIT: place an ad in the Parts Wanted forum and I'll bet someone will offer one up to the stovebolt gods.