I can't speak to a 56 box but I've rebuilt 55's and 57's. The seals were off the shelf items with either a stamped mfg # that the bearing store could cross reference or simply match by diminsion........Bob
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Bob Beck
39 Chev PU
69 big block Corvette
55 Buick 66C
57 Buick 46C
55 Olds S-88
56 Chrysler St. Regis
AACA, BCA, WPC, USHGA