All Early Thunderbirds have an electrical ground strap connecting the engine to the body of the car. Since the engine is mounted on rubber this connecting strap is vital. It is located on the left head at the rear of the engine (or at rear of block next to the left head) and is then attached to the fire wall. Check for corrosion or damage and replace if necessary.
On the 1955 & 1956 Thunderbird it is attached under a bolt on the left head. Use as short a bolt as possible; if the bolt is too long it will press against the push rod for the exhaust valve on the number 8 cylinder. I've seen more than one push rod worn almost all the way through.