The mounting plate bolts are carriage bolts because the plate is stamped with a square hole to accept the carriage bolt because it prevents the bolt from spinning while tightening.With the fan and radiator in place space is limited for an open end or box wrench to fit the hex bolt head. If the radiator is removed, you'll have plenty of room for hex head wrenches. The original carriage bolt size is 3/8"- 24 x 1 3/4" and drilled for a cotter pin. Replacement carriage bolts don't have to be exactly that unless you're going for a pure restoration. Just use a 3/8" carriage bolt of the same size with a course thread and a nylock nut.

Drew