A 12V starter will not mesh with your flywheel if still has the 6V flywheel in it.

Another tip. If the replacement cables are the same diameter as modern 12V cables they aren't big enough for 6V.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.