i get the impression the o.p. Wants to make a b-body od trans out of parts from an a-body od trans. So the end product will be a b-body od.
If so, the o.p. Needs the gears from the od trans and an od main shaft in b-body length.
The main shafts are different from a 1 to 1 trans and an od.
The a-body od main shaft is too short for the b-body trans, however brewers has them in stock.