As it turned out, I did go back for the original engine. Mainly because the engine number is on the title not the body serial number, good thing now at least I can prove this truck and its title is authentic. Thanks for the responses.
1953 Chevrolet 3100261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY GalleryVideo of the 261 running1964 GMC 1000305 Big Block V6, sm420, the next cab off restoration