The pump kit #763758, that you referenced does not fit the 55 fleetwin 7.5. The impeller bushing bore is too large for the shaft. I suggest getting the original impeller p/n, and clean the cast housing up and reuse it.
I have a 1954′ Fleetwin that had one of these kits, and the impeller fit fine to the driveshaft. The lower unit of a 1954 Fleetwin is mechanically identical to a 1955 Fleetwin. Within the 1950’s OMC classics, the next size up in waterpump ID would likely be for a 10 hp or larger motor.
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