The difference from '64 to '65 is the ohm resistance of the sending unit. The '64 is a 30 ohm unit and requires the matching gauge . The '65 is a 90 ohm unit and uses a different gauge then the '64.
I've recently replaced the sending unit on my '65 GS with a new stainless one. First time the gauge has read correctly in 28 years.
If Chuck doesn't have a sending unit, I probably have one for a non- AC car.
Yachtsmanbill, that new sender is not going to read correctly in your '64. If you change the gauge in the dash to a '65 piece, it will be fine.
Loren