[Captkaos]

1979 was the first year for the filler door, 1980 was the last year for the tank to be standard on the passenger side, 1981 it moved to the drivers.

This ONLY applies to pickups, and only single tanks:
73-78 had exposed filler cap on passengers side
79-80 had a filler door on the passenger side
81-98 had filler door on drivers side.

Burb/Blazer had filler doors starting in 1976, from memory.