The front section has to be shorter. Then you can get into differences in the flange sizes, Volvo had (at least) 3 different sizes,a nd seemed to switch between them almost at random. And the flanges off one trans won't necessarily swap to another trans.
It can be somewhat of a minor headache.
When I put an M41 in my PV I got lucky and my new late model M41 came with the same size flange as my PV's M40, so I just measured carefully, and took the driveshaft to a drivetrain shop and had them chop some 11-ish inches out of the front section, reweld and rebalance it.
It's somewhat painful to drive one of these old cars at modern highway speeds without an OD 5th gear. They'll do it in 4th, but that motor is *spinning*.