Flywheels can be resurfaced if not severely cracked.
Flywheels with 139 teeth are made for 6 volt starters.
A 6 volt starter motor is a torque motor and operates by current and will work okay on 12 volts.

From Chevrolet Master Parts List (1929-1957):
Original clutch disc is 10 inch 3836119 for woven material.
Then in 1957 the 10 inch disc 3836097 for molded material
Pressure plate 3704776
Clutch cover with diaphragm springs 3836716
If I did this correctly, your FLAPS should be able to cross these GM part numbers over to what they sell.
If they don't know or cannot help, find a better supplier.

The diameter of a pressure plate is by design larger than the clutch disc.

I'm not much help on T5 at the moment.

Attached is a 1956 Service Bulletin which also includes trucks.
Maybe the pictures are helpful.
Attachments

"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."