Here's a pic of the engine harness where it goes through the firewall:
There is a similar looking (and slightly larger) bulkhead connector above the fuse box that has 2 plugs on the under-hood side for the generator/alternator/headlight and front parking/turn signal wiring harnesses.
The cab harness side of that connector (and grommet) looks like this when it's removed from the truck:
Here's a firewall grommet set that includes both of the wiring harness bulkhead connector grommets along with some others -->
http://www.classicparts.com/1960-66-...ctinfo/42-805/