I need some help with an old Dodge truck.

6V systems DO crank slow. They are all low compression, so start easier

For voltage, just dived everything in half. A fully charged 12V is about 12.6, so 6.3V

12V "running" are an nominal 13.8--14.2 so 6v...........about 7V

For conversion...........

Most electric motors (back then) do not care about polarity. NEWER permanet magnet field motors (like mini-starters) DO car about polarity. So if you have the original heater and wipers just use a dropping resistor. You must size them as to DRAW, so hook them up to a hot 6V battery --or power supply--and measure operational current draw

Gauges need a special drop for each gauge. I cannot remember the name there is/ were little modules you could buy for each one

If you reverse polarity, reverse the wires on the ammeter

Use a 12V coil or a Mopar style coil+ matching ballast combo

Usually you can "get away" with the 6V starter and even the 6V solenoid if used, but many of those old girls had 12V starters available, and I would check into that. You also "might" find a by Gawd honest to real old school local rebuilder who can turn a 6V one into 12 (rewind it)

Edit gauge drop search might get you started........

6 to 12v gauge instrument drop - Google Search