I RECENTLY HAD THE SAME PROBLEM WITH A 6 VOLT STARTER I HAD PUT ON A 1955 235 THE DRIVE GEAR WOULD SLIDE OUT TO ENGAGE THE STARTER BUT DID NOT FIT THE FLYWHEEL.

AFTER COUNTING THE TEETH I HAD A 168 TOOTH FLYWHEEL SO I GOT ANOTHER USED STARTER OFF A 235

AFTER COMPARING THE DRIVE GEARS, ONE HAD A SMALLER DIAMETER DRIVE GEAR

AFTER INSTALLING THE ONE WITH A SMALLER DRIVE GEAR IT WOULD ENGAGE BUT NOT SPIN THE ENGINE.
TOOK IT BACK OUT AND TO A AUTO ELECTRIC REPAIR SHOP. IT WAS TESTED AND THE DRIVE GEAR WAS BAD, COST ABOUT 20 DOLLARS TO REPAIR. THE REPAIR MAN SAID THE DRIVE GEAR ALSO ROTATES ON THE OUTWARD STROKE BEFORE ENGAGING THE FLYWHEEL... WHERE THE OTHER STARTER WITH THE LARGER DIA. GEAR WAS JUST A STRAIGHT OUTWARD MOTION WITH NO SPIN BEFORE ENGAGING THE FLYWHEEL

NOW MY ENGINE STARTS FINE { JUST REBUILT }
BUT I GOT SOME PLAY IN THE DISTBUTOR
I HAVE 2 LAYING ON THE BENCH.. I'M GOING TO GIVE THEM A TRY BEFORE GETTING A REBUILT UNIT