Might be a good idea to learn about simple electricity someplace to learn how it works, pretty simple, just a few basics would help.
Ammeters do not care about voltage, just flow, so, if one turns on the headlights or some other load with the engine off, the ammeter should indicate negative, if reads positive, just change the leads. The whole electrical load less the starter, should feed through the ammeter, lead goes from the starter large starter terminal directly to the ammeter to feed the whole rest of the truck, which gets fed from the other ammeter terminal.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.