273 crank in 318 bennifts??

A cast crank can hold a lot of power without worry. Do you plan on going north of 500+ HP with the cast crank? Then go forged if so. Not that going forged to start is a bad thing.

The only thing I can think of with the 273 crank is IF there is a difference in the bolt holes at the rear of the crank. 6 evenly spread or off set holes? 4spd usage would have the need to look at the pilot hole for bushing use and depth for the manual tranny's input shaft.