My 65 Valiant ammeter melted down

Yes, you can keep your ammeter functional but still protect it with a bypass diode. Either this, or the "Mad Bypass" is mandatory if you upgrade your alternator. Even if you make no changes, clean the ammeter terminals and check the insulating washers.

The dual diode shown below was $13 on ebay. Cut in half, flip one around so the pair conducts both ways and bolt together. Put it in-line with a new 12 awg or bigger wire from the alternator output to the battery. Install at either end, but it needs a good heat sink. Calculate the power dissipated as P = I x dV, where dV = voltage drop across diode at current I. You can find a dV vs I plot in the data sheet (web search). I recall ~0.5 V drop at 100 A, so 50 W of heat. A 0.5 V drop will put your ammeter about 75% scale. At low currents, most will still flow thru your ammeter.

Read more here:
http://www.forabodiesonly.com/mopar/showthread.php?t=214956&highlight=diode

PM me if too overwhelming to buy and fab yourself and I might sell a spare setup, with no guarantees since I haven't run this latest incarnation yet, and I would charge well for the fuss.