what fits what

i have a 41U will a transmission from a 41 knuckle fit or are the main shafts different lenghths.i see trannys for sale but they are listed as 36 up.since the knuckle came in 36 i figure they will not fit a flathead.

'36- '64 all B.T.trannies will fit and even later ones can be easily adapted.
Stéph

thanks,you guys are a wealth of information.