Hi Chris,
So if I am understanding, the main complaint is a slow crank when the engine is warm. Correct? Sounds like you have a new and tested battery, swapped out the starter from an old to new one, then back to old again, and you have 2/0 battery cables. 2/0 cable to starter too I am assuming. I am wondering if you are dealing with a voltage drop issue. Have you tried changing out the starter relay? If it's old, due to age and use there could be high resistance contacts inside of it. If the contacts are worn out or burned they may be developing a voltage drop when you are trying to crank. I would think that if you tested for voltage drop from the battery all along to the starter you could find your culprit. But if that starter relay has not been replaced with the starter, you might try measuring for voltage drop across it first while cranking. How much voltage drop should it be? I would think it should only be 1/2 volt but should be probably less. A quick test would be simply just take an old battery cable to short out the starter relay. Temporarily jumper the cable from the battery to the cable to the starter. (hope you don't mind sparks flying. Use gloves and safety glasses) If the starter turns over faster, you found the problem. Also even though your cables may be new, there could be a chance one of the terminals may not have been crimped correctly, thus maybe also giving you a voltage drop on cranking. If you could find a '54 manual, it will take you through a similar procedure for slow craank and will spell out the approximate voltage drops expected through the starter circuit.

 

You need to perform a starter draw test. This procedure will test every component in the system.

When the engine is warmed, the starter turns slower. Too much AMP draw.

GOOGLE - STARTER DRAW TEST

BTW - Two good suggestions posted -holding the throttle slightly open (choke usually does this) and holding clutch pedal in while cranking.