Ammeter to voltmeter conversion

Idaho fish said
I am getting ready to order a wiring harness kit for my 68 barracuda. I am finding out I need to change from an ammeter to a voltmeter.

Who told you that and why do they think you need to do that?

Idaho fish said
How do I do that while keeping the stock gauge and have it read correctly

If you are asking about keeping the Alternator (ammeter) gauge face plate but with a volt meter guts, that has been answered already above.