Try Push/bump starting it with the starter out, See if it grinds then. The grinding I have only heard of once or twice, once when the ring gear on the flywheel (std tran) shifted and was rubbing the starter housing. The other was a stuck starter solenoid holding the gear against the flywheel teeth. Or ... wrong starter installed, which does not sound like your issue. Also check the voltage AT the starter, Key on, and test both POS wires for 12 volt, also ground to engine (clean and tight) same with POS cable.
Thats all I can think of for now, get back with us and tell us how it's going.