If you have a 1948 Thriftmaster, most likely you have a non-original engine. Do you have a newer, high-pressure 1954-63 235?

If so, you will need to use the harmonic balancer off a 216 or off an earlier 235 that have a hand-crank fitting on the front of the balancer.

http://www.chevsofthe40s.com/detail/13746/Chevrolet_HAND_CRANK_MANUAL_START_HARDENED_STEEL.html