Doc here,

imp:
The first thing you will need is a 12 to 6 Volt regulator For the gauge power. You Can buy these, ranging from $50 on down, or build your own for under $5, or scavenge one out of a 70's Ford or Dodge in the Bone yard.
If your gauges are different from stock NOW, you'll need to find the resistance value of the senders you have now..These must match the resistance value of the stock gauges for them to work and/or be accurate. So those may need changing.
If you have an AMP gauge , and are now running an Alternator, It is not going to work (Very long), as Generators were -30, 0, +30, and most Alternators are 64 amps or higher under full load. Additionally if the vehicle was positive ground, and you do try to use it, it will need to have the wires reversed, or it will indicate backwards..Best bet there (and much safer) disconnect it, and use a modern Volt gauge.
Don't forget to upgrade the panel lamps too.
Doc

imp: