Slant six was running great now I don’t have electrical power

Thanks for everyone’s help. Really appreciate the support from this forum. I checked starter relay and it is fine. I have a bad bulk head connection. Wiggled it around and it works in one position. Are these easy to replace? Or do I have to replace entire harness from bulkhead forward into engine compartment.
Disconnect the battery.
Then remove the offending connector. Look to see what is wrong.
Follow this link for some advice on removing those connectors.
question on the main harness plug on firewall
Yes you can replace just the plastic connector and the foam seal.
The next link gets into removing and cleaning terminals.
1969 Dodge Dart - Engine Compartment Main Bulkhead Connector