First thoughts the best

I have bought a 1972 XY Ford Falcon ute with a 221 cubic inch motor and a four-speed gearbox. The motor replaced the 250 cubic inch original motor. I have another 250 motor that I thought I might put in and get the valve seats hardened to allow it to run on unleaded fuel. But someone suggested it would be better to put a 3.9 multi-point engine mated to a C4 three-speed auto box from an EA Falcon. I believe this motor would fit into the engine bay without much alteration. Would you know if this advice is sound? Or is it too costly to obtain these and get them fitted because I couldn't do the job myself? Could you suggest other, cheaper options for the engine/gearbox to allow the car to run on unleaded and fit into the engine bay without much rearrangement?