The Gates 9365 represents a belt 9/16" wide by 36 1/2" long.
A "correct" belt would be either 3/8" wide by 36" long or 7/16" wide by 36 1/2" long.
If you notice, each 1/16" increase in width means 1/2" increase in length.
The wider the belt, the higher it rides in the pulley.
With that in mind, I'd say a 9/16" wide belt would need to be 37 1/2" long.
Or see if there's a Gates 7365(7/16" wide)
Don't know if this will make getting the ps cap off any easier, it's a pain pretty much every year C3! And 80-82 used a different alternator bracket setup that may make it even harder to access the ps.