If you are going original here are some things to check before you spend the $$$:

HT = Hydramatic Transmission
Engine and HT may be a matched pair (some HT have engine serial number stamped on a HT tag).
Engine may be stamped with code letters indicating HT. (http://chevy.oldcarmanualproject.com/models/engine.htm)
Using another engine may require checking the alignment of the housings and check the flywheel and torus cover run-out.
(https://www.stovebolt.com/ubbthread...ans-front-rear-housings.html#Post1355625)

Make sure you have a good/correct VIN tag (door post) that agrees with your title.
VIN stamped on frame (https://www.trifive.com/forums/showthread.php?t=115637).

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."