Using a multimeter and a visual check of wiring, try to track down damaged wiring. Disable the anti-afterfire solenoid if needed. That stupid thing has to be working correctly or it will never run.
I hope you mean the the hot wire on the plug reads 6.5V .
This will be because of a bad or dirty connection
It has a plug behind the starter motor and the only other one is at the key switch
Back probe the socket at the key switch, check for full battery voltage and at the plug near the starter
They do get corroded and that causes voltage drops .