Hi Tom,
It was a bolt in, the newer trans was in the truck when I had acquired it 4 years back. A few years back I had installed a new clutch, and the 3 speed trans slide right back in.
The trans should be the same, since any engine & trans combo's could have been ordered from the factory.
Even a 4 speed muncie, or saginaw 4 speed(car) out of a chevette should also work too.
The SM420 (4 speed) that was attached to the V8 '60-'62 bellhousing had the 2 lower bolts go from the inside of the bellhousing into the trans, and the top 2 were screwed right into the top of the bellhousing. This was attached to the V8 I had picked up from another board member.
If your truck came originally with a 3 speed, the slip in yoke should go in, but on the SM420-SM460 (4 speed) has 2 "U" shaped with a "U" joint that bolts directly to the back of the 4 speed. If your truck has this setup, a new driveline will be needed.
I also forgot to mention, the 4 bolt holes on the 3 speed are perfectlt uniformed (top & bottom row), I forgot what the 4 speed(SM420) trans bolt pattern was.