Real quick,
GT4 (3.73) and W42 (442 option) MUST be on the RPO sticker in the trunk. It'll have two colors noted either on the cowl tag or RPO trunk tag, one of them being silver (lower). And obiviously gold stripe package. Comes in Gray, Dark teal blue, black, or burgundy on top color.
15" wheels, 215/65/15 Goodyear Eagle GT ROWL tires. Check the tire placard decal in the door jamb- should show the tire size. If it has G80, it's a posi, but G80 was an option on all
H/O and 442 G-bodies. No vinyl top ever. Reclining bucket seats and console shift with 307 and OD transmission. Dual outlet exhaust.
Also, 8th digit of the VIN MUST BE a 9 with a check digit to follow, then the year, G for '86. Will be a Salon body made in Pontiac. So the VIN will look like this : 1G3GK479(check digit goes here)GPxxxxxx
This isn't EVERYTHING. I could go on for a good while.