A 5.0 in a '50 Ford sounds like a winner with a C4 or earlier mechanical AOD even with trans tunnel work. The diff will not survive long with the added horsepower of the 5.0 but an 8" would be fine (been there.....). There are about as many gear choices as OEM 9" units without the big bucks as most salvage yards keep crushing the 'little' ones to build even more Toyotas. (or think about a 351W/5.7 as another slightly more interesting build)