The O/D gears will fit in the early, 3.09 first gear, cast iron main housings without modifications. Maybe all the early main housings as I doubt they would have made two different castings.
Well they wouldn't fit in any of the 4 cast iron boxes, I tried.
Oops, I think I read that wrong.My cast iron boxes are 1969 and newer.So perhaps there are two cast iron boxes. Maybe Chevy used the early boxes to stuff the o/d gears into for their pick-ups