['ol Pinion head]

Best way to know for certain is pull the rear cover & look at the inscribed numbers.

My Dad special ordered '76 & 77 SJ's, both near identical, Silver exterior/carmine dash & door panels with white bucket seat interiors/AC, tilt, pwr windows, pwr locks, cruise. Both were 400 cars.
The '76 had 2.93 gearing, the '77 had a 3.23 STT. Figured out the '77 gearing shortly before Dad sold the 77 SJ & received the '79 403 OldsmoBird, he'd ordered for my mom. For some reason, Dad thought the '76 had more get up & go than the '77, but his idea of performance was out on the highway, when he stomped on the pedal to pass another vehicle.

2.73 was also a popular ratio in the '77 GP's. If the '77 GP was built with a 301 4bbl, trying to remember but believe it received 2.41's. In the 90's through early 00's, I pulled the covers on a ton of '73-77 8.5 GP rears in Pick-N-Pulls & crusher yards. Pulling bolt in axles, posi's, & select ratio used 8.5 gear sets. Odd enough, have pulled 3.23 gears & posi's as well as tach & gauges & clocks out of a half dozen '77 GP's, only one set of tach & guages was from an SJ. The rest were out of LJ's.