When I changed my 52 fleetline to 12 volts The only gage that required avoltage drop was the fuel gage. The oil press is mecanical so is the temp. the amp gage will work fine on 12 v. I believe your 48 is the same. I made my own 6-V voltage regulator with radio shack parts. I could send you schematic If you would like. Easier would be finding a gas gage voltage regulator from the junk yard or parts store. Most modern cars use 5-6V for the fuel gage. My system is still working fine after 2 years. I still havent changed the dash lite bulbs. I just keep the dimmer turned down low.
Best of luck on your swap Dawger