A 30A is likely not enough, when charging hard the line will be flowing more current. Might get by for a short time
The ammeter in these girls has everything to do with the ENTIRE electrical system, except for the main starter power
You need to get yourself a 74 factory service manual. There should be one "coming up" at Mymopar as I sent him one. Meantime the electrical system IS DIFFERENT than other years, and I've posted alink to a reduced file for ONLY the electrical section of the manual, here:
Free service manuals
Go to post 64 on page 3. Click the black icon in the box and 'save as' Its a pdf
Pleas go here and read this article which points out the problems with these girls ammeters
Catalog
On that page is this simplified diagram of main power distribution
View attachment 1715785658
Notice the "welded splice" in the large black ammeter wire. This is under the dash, not far from the ammeter, wrapped up in the harness. That splice branches off and receives power from the alternator, and feeds power to the headlights, the fuse panel "hot" buss, and the ignition switch main feed. Some years, one or two other feeds.
Since you have all this apart, I would carefully take the ammeter apart, "if you can solder" and silver solder the studs to the shunt. You will need to get this all clean and use proper flux. Some of us bypass the ammeter and convert to a voltmeter
