The 46 front motor mount can be used on the 58 engine. In fact a Chev front motor mount/cam cover will work too. Your flywheel is probably a 4 bolt on the 46 and a 6 bolt on the 58. The 58 will have the smaller teeth designed for the 12 volt starter. 6 bolt 6 volt flywheels are around, but are scarce and therefor expensive. I know a guy that puts 12 volt field coils in a 6 volt and could probably do it the other way around. Do you have a starter on the 58 engine. It will be a different tooth count than the 46.