Ammeter to Voltmeter...who does it?

If i had to guess, the circuit breaker is there because an ammeter measures the current flow. Too much current flow and it breaks the circuit before any damage can occur. All your alternators output power flows through the ammeter before going to the battery. By contrast a volt meter is only going to take a sample reading of the voltage level and not current or amps. Not sure if this makes sense to you. I would assume by changing over to a volt gage you no longer need the circuit breaker at the gage.

Your van cluster ammeter setup is similar to the 67-71 rallye dash cluster in which it has 4 instruments in one location. Only difference being the A body rallye cluster it faces up. Not a big deal if your volt gage has a potentiometer on it, you can fine tune the needle adjustment where you want it to sit at a given voltage.

How i adjusted the one for my barracuda was i took a car battery and set it on my workbench and measured its voltage. Then i hooked up the volt gage to the battery with alligator clip test leads, and oriented it in my hand how its positioned in the gage panel, and adjusted the needle position with the potentiometer. For me 13 5V is slightly above middle line towards charge on my ammeter.

The circuit breaker you would need to reuse however. I would put it somewhere between your battery positive post and the alternator output wire going to the battery. Maybe a convienent location on the firewall.

When you hook up your volt gage in the instrument cluster, i recommend grounding your negative gage post right to the metal cluster housing, and enlarge the positive post hole for a plastic grommet and plastic washers to insulate it. Find the 12V feed that goes into the cluster from the body wiring plug. And trace that to the IVR input side, and run a wire tap off of there to your 12V input to your volt gage.

I noticed your gage face does read opposite of the A body cars. It may be a simple solution. Theres companies out there that make repair decals for vehicle gages. They may have these for dodge vans, and may be able to make you up a decal thats volts instead of amps, you may be able to explain to them that the scale on the decal needs to be opposite of whats there.

IVR (instrument voltage regulator) is the rectangular tin box with prongs on it plugged into the cluster.circuit board. It regulates 12v down to 5v for fuel level, water temp, and oil pressure gages. Its a set of vibrating points. They worked great when new but get weird when they get old. If it sticks open the gages drop, if it sticks closed the peg out.

There are 2 solutions to this problem. You can buy a plug n play solid state one for about $30. Plug it in and its done, or your can wire up your own with an NTE960 semiconductor and a 100 microfarad capacitor for about $3. I went the $3 route because chrysler stuck the IVR inside the fuel gage on the rallye dash, and i had to dissassemble and gut the IVR side of the instrument while leaving the fuel level side of the gage intact. Since your is plug n play on the back of the circuit board i would recommend just buying a solid state replacement, plug it in and done.

I hope this helps you out a bit. Its hard to see what would fit the cluster you have without taking it apart and seeing what it looks like on the inside.