Installing a solid state 7805 device is described in our very own Clubhouse at:
http://1962to1965mopar.ornocar.org/ml-instrument-voltage-regulator64.html
If you're comfortable with simple soldering of wires to a 7805, you can do the conversion for a couple bucks. Otherwise, as mentioned, the prefab units from RT Engineering look very nice. They're sold on eBay -- search "Mopar solid state limiter".
If you have a Mopar with points buried inside the fuel gauge (like we had in our 64 880), you'll have to remove the gauge and do a little trickery to bypass them and use the solid state voltage limiter instead. The easiest thing is to cut the output from the points and connect it to the same wire that supplies the water temp gauge. Both the gas gauge and the temp gauge won't know the difference, except for seeing clean 5v DC instead of chopped up 12v that's trying to average out to something like 5v. (Put a meter on the gauge points output and you'll see what I mean.)
The description on RTE's eBay item page is also a helpful read.
Dan