‘66 Cuda Won’t Turn Over - Only Got 6V at Switch

I did not change any part prior to the problem. After the problem I did put in a new relay, changed ignition switches (had a spare). I have full 12.6 on both sides of the fusible link, although it gets hot when key is in “Start” position.
Also did pull all three big plugs out of the firewall. None look burnt. Took a fine file to clean all the male terminals, hen plugged all back in.