They did indeed come with a V8 option from mid 54 on BUT that early motor mounted off a bracket attached to the front of the motor and had a transmission mounts on side of the bell. That stuff isn't too compatible with the modern engine and transmission mounting systems.

You will likely find that you will be farther ahead to build brackets off the frame (include both the top and the bottom frame flanges) for the motor, and then add a transmission crossmember at the rear end of the trans (if there is not already one there). I had an original V8 54 Dodge pickup (one of the first V8 Dodge pickups in our area), but it was missing the motor & the mounts. The two brackets for the original bell mounting were in the way of easily installing a modern 360/727, so I cut them off at the frame and built motor mount brackets and added a transmission crossmember. My motor & trans sat back a little more then the original V8 did, those original motors are mounted so the motor is centered above the front axle. Setting the motor back required modifying the firewall quite a bit, for engine clearance. I suspect that by 59, that issue may have been solved, my truck was a very early factory V8 truck. Gene