If it's the original 6 volt starter it should spin the engine very well on 12v.
I've never used one of the PowerMaster starters and they may be fine. If they're made in China having the existing one properly rebuilt may likely outlast it.
There are Six battery & ground cable connections that must all be in very good condition for the starter to turn the engine easily. Both ends of the battery Positive cable, both ends of the battery Negative cable and both ends of the solenoid to starter cable.
If any of these is in poor condition it can easily affect the Starter power circuit.
Assuming the car is now Negative ground... the other end of the battery Negative cable should fasten to a clean bolt surface into the block or head, not the car body.
If the starter were still in the car, you can use a set of jumper cables from the battery to power the starter directly. This eliminates the firewall / fenderwell mounted starter solenoid as a possible cause of a poor connection.
Original style parts generally fit better overall. If a local shop can give you a better or similar price & warranty on the rebuild than the PowerMaster that sounds like a good deal to me. (and no shipping charges)
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