[Lynn Eades]

To add to Bruce's input. The voltage going to the coil is the full 6 volts (battery voltage) This is to aid in supplying good voltage to the coil at start up. As the resistor heats up, so does the voltage (to the coil) drop to the 4.8? (4.5?) volts. The ignition resistor has a tin cover with holes in it (the cover is to stop you burning yourself + other things. Make sure your gauge resistor connects in from before that.
The problem with diagnosis via internet is that those attempting to help usually don't get the full details, and so the chances of getting it right are a bit like trying to win lotto.
Having read a bit about faults in Jeep fuel gauges, the gauge body has to have a good earth in the dash panel. In turn, a part of the gauge circuit needs a good earth inside the gauge body. These are common faults in old Jeeps. There are many things to consider.
Tom, the resistor you are using, Is it matched to the gauge? What I mean by that is that the gauge needs to receive a 6 volt input while the load (the gauge) is live (operating)
Does your carrier have 1 sender or 2? (one in each head?) In your left bank you should have one with a single terminal, and in the right bank you should have a sender with 2 terminals.(wired in series) The single terminal one does a gradual signal while the 2 terminal one is just a "hot on" "cold off" switch. I dont think Ford made a separate single terminal one for a 12 volt gauge (matched pair) but your original gauge is 6 volt so you have to use it that way (with correct resistor)
The system (less the resistor) is covered in the info that Tony directed you to. If you cant sort it out your self, then point the info to your 50+ y.o. auto sparky.
The Ford v8 is like 2 separate engines and the water only meets in the radiator. If you took the two wires off the R.H. sender and connected them together and insulated the join, then you could pull the sender out of the R.H. head and fit a separate bowden tube (capilliary) type gauge to that bank. It would then give you an idea of what's happening.
Lastly, you want the engine to run as hot as possible, but at the same time, to never have it boil. Good luck!