to clarify, H41 is low gear first tranny and comes in many varieties. First made in 1967 and last in 1989 or so. 5.3, 4.9, and 4.8:1 ratios- the 5.3 are pre 1972/3 very, very rare.
H42's are mostly 3.55:1 ratio and made 1974 (ish) to 1989 (ish).
your question: will a early main shaft fit a...