64 valiant gauge problems

This is a delicate matter and you can easily screw up the gauge. I'm not intimately familiar with these, but if you Google, there are photos on the www. Basically it involves removing the gauge, bending/ cutting the old unit out so that it does not short with the fuel gauge, and then either adapting a late replacement (external, non Ralley) or build your own. There are also several projects on the www concerning this.

Otherwise, find someone who commercially restores the unit.