There is no difference in the heads due to the internal/external bypass issue, it's all in the block or external bolt-ons. There is an extra 0.400" hole adjacent to one of the three bolts that fasten it to the block, which feeds water from the block to the center of the waterpump for the internal bypass. The external bypass is in the thermostat housing on the head, feeding water into the heater connection on the top of the waterpump.
A word of caution; don't use the external bypass parts on an internal bypass motor. Can you guess how I know this?
Marty
