Isn't the '61 a coil spring rear suspension?
You'll have to cut off all the coil spring stuff and weld on leaf spring perches to put in your '50.
Or...you could just install the '61 center section in your '50 rear end. It's a bolt in deal.
The '61 rear will have better brakes though.
As previously mentioned the '61 is probably 3.90.
I assume the '61 is a half ton also.

Steve

'49 3800
'52 3600
'57 3100