I think I have the same problem as the OP and followed Steve's link to the RockAuto part. Then searched for the starter drive for my '57 235 and hell they list about 6 or 7 possible. Since they show OE numbers, I dove into the GM parts WIKI. Page 204 of the March 57 book Wiki lists the starter drive as 1927200 which goes to this one

Just saying one linked, might not be right for all, but that link did lead me to something to try for mine. :-)

Eric
http://s1199.photobucket.com/user/sparky234/library/My%2057%20Chevy%20Stepside?sort=3&page=1
'57 Chevy 3100 (Little Red)
'81 RD350 LC - Fun, Fun, Fun
'83 GS1100E - Pocket Rocket