Mickey - The original seal is GM# 3746212. The same seal is used for the standard 3 speed in your truck plus it also fits Powerglide transmissions from the same time period. You can find a few NOS seals offered on eBay as well as other vendors with aftermarket replacements. A couple of our usual vendors are listed below.

The Filling Station sells them as a Powerglide Rear Seal

Jim Carter offers them as a 1955 and later 3 speed/Powerglide/Turbo 350 rear seal under PN: ME505
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)