Does your heater motor happen to have three wires by any chance?
The 12 volt heater motor that is in my '50 has 3 wires. One is a ground wire, the other two are positive wires. One of them causes the fan to run like a jet engine. The other one runs it at a reasonable speed. I just hooked that one up and left the other wire disconnected.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission