Help, removing front wiring harness from straight M151

[M37]

I am stripping the body and I am removing my front wiring harness on my 1962 M151. The cannon plug on the #5 wire (wire that screws into the regulator) won't fit through the hole in the firewall.

It appears to me that the cannon plug needs to be disassembled/unscrewed, in order to fit through the hole in the firewall. Is that what needs to happen?

I've soaked it in Kroil, but the back half won't unscrew from the big front ring (part that threads into regulator).

Thanks for any advice or help.