I found the correct 7/16 carriage bolts for the front engine mount on Amazon. They were zinc (or maybe cadmium) plated. Unless you want to drill them for cotter pins, I would use Nyloc nuts on them (or another prevailing torque style nut.) 7/16-14 is coarse thread and 7/16-20 is fine thread. Either will work, but fine nuts are probably harder to find, as 7/16 is a fairly uncommon bolt size.
What's interesting is that the 1929-1954 Parts catalog doesn't even list 7/16 carriage bolts, but does list 7/16 nuts in all types (including the castellated lock nuts) in both coarse and fine thread.
