I had a toploader OD behind a 351M, worked great. Just be sure to get the iron Orion box with external shift linkage, the SROD aluminum version with enclosed linkage is ****.
Most 351C engines used the Windsor (302 and 351W) bellhousing pattern. This means that your OD trans options are virtually unlimited. As already mentioned, You can us a Borg-Warner 3spd + OD out of mid 60's Full Size Car. The 4 spd RUG OD out of a late 70's early 80's F-150. You can use a T5 out of a Mustang, or a M5R2 5 spd out of a mid 90's F-150. As far as auto's, The venerable AOD is you best bet. You will also be able to easily hook up after market Tremecs or Richmond's if you have deep pockets. Also, check out Quicktime and Lakewood. They make bellhousings to put just about any trans imaginable behind a windsor pattern. If by some strange reason you have one of the odd ball Cleveland's with the 385 big block series bellhousing pattern, then you are limited to larger truck 5 spds and automatics from the 90's or the aftermarket Tremecs and Richmonds. Good Luck.
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