Why don't you convert to modern mechanical gauges that will fit your engine, that you don't have to voltage reduce and you don't have to adapt 6 volt electrical sending units that are expensive to buy - if you can find them.

Adapting 3 1/2 inch Supro mechanical gauges into your existing gauge cases and using the stock faces is a relatively simple process that only requires some minor cutting of the stock gauge case.

Look in my Galleries. There is a gallery that tells you the step by step and has a picture of each step.

I have done this with my oil and temp. Ammeter does not need to be modified, and gas gauge can be voltage reduced.

Here is a picture of my gauge package after the conversion: