I believe the 1961 truck came with the torsion bar suspension. I have a 1966 c10 and it's set up from the factory to accept a 230-292 straight six or any small or big block v8 - only the 292 requires a frame bracket change.

The '66 has a crossmember in place in which to mount the engine brackets to the frame. Being that a small block was optional back then you should be able to mount a 250 by just finding the appropriate v8 hardware from another '61. Since the 292 mounts differently on the passenger side, and wasn't available until 1963 chances are that some fabrication will have to be done to the frame and/or crossmember to accept the passenger side bracket.

-ftyler