I don't understand what mean by gutting the 6 volt motor.All you are doing is removing it.
The 12 volt motors from Classic parts of America have the 5/16" shaft,no adapter collar needed.Part # 68-009
blower motorIf you purchase this motor,orange wire is ground.If hooked up wrong,it will spin the wrong direction.
Just center the new motor and drill two new holes.The new motor will require you to cut a new rubber gasket for heater motor to firewall ring.The new motors are slightly smaller in diameter,easy to do.
You can purchase the push on keeper at your local hardware store.
Good Luck !!
Tim