1955 to 57 passenger had no removable differential cover.
Possible ratios:
https://www.trifive.com/threads/rear-axle-ratio.38505/

If this is a trifive rear axle it may not be a good candidate for today's hotrod.
It will not last long with a performance 350 engine and 28" tires for example.
It's about 66 years old and probably still running on the same bearings.

Truck rear axle info:
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=656825

"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."