Stupid Ground Question

My 68 Dart currently has a ground cable from the battery to the front of the drivers side cylinder head along with the small pigtail to the radiator core support. There is no ground wire or strap from the rear of the engine to the firewall. My questions are:

1. Is a ground strap/wire necessary from the engine to the firewall if one exists from to the radiator core support?

2. If so, what size wire/strap is adequate for this?