Mounting holes for the 6 volt starter are 2-1/4″ spacing and mount holes are 12v starter are 2-1/2″ spacing. If you can find a 1957 bracket, part #376885, it will have 2 sets of mount holes so you can mount either starter – see pic. Easier to go with a 12v starter and battery and as Frank noted, use a resistor for the choke solenoid.

Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1955 Johnson QD-16
1957 Evinrude Fastwin 18
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."