There may be two seperate issues.
Reasons are this
1. That little black doesn't seem like the result of something that created noticible smoke. It looks something that sparked.
2. If the short to ground was there, or anywhere in the battery connected lines, it would have smoked when the battery was hooked up.
My understanding is the smoking and smell occured when the key was in run (or maybe start). That means the connection that smoked was downstream of the key switch. Something or some branch of the run or accessory circuit.
Possibilities:
One issue may have been no battery power getting through because the connection at the ammeter was loose (nut stripped or not tightened enoguh)
A second issue is something shorted or overheated (another loose connection) that still has to be found. But it’s most likely fed by the run circuit, possibly by the accessory circuit. Since the fuel gage smelled bad, probably worth looking at next.