I have pulled a 250 out of a 1949 truck that someone had transplanted in. It had home made engine mounts that reached out and sat on top of the frame. The cross member had been cut out from the location where it normally supported the rear of the motor and the cross member that is normally supporting the rear of the tranny had been modified to support the modern transmission that had been put in. It all fit OK - but with the cutting on the mid cross member, I wondered about the rigidity of the frame.
Easiest swap by far is a 261 . . . not a bad motor and set up right it will move your truck smartly down the road.
If you want to go with a 250, the easiest approach is to get all new bellhousing / transmission along with it and then do a quality job of fabbing mounts for the motor and the tailshaft of the tranny and dispense with the idea of mounting the aft part of the motor/bellhousing to anything.
