Here's a pic of the seat you are looking for:
They are not as common as the 60/40 split bench. You can look in regular cab trucks from 95-98(00 for 3/4 and 1 ton). You can also look in the rear of a crew cab from 95-00.
My 99 crew cab had a seat like that in the back. And my 96 c3500hd reg cab has one in it too.
Most 95/96 trucks just have a fold down armrest without storage. Most 96+ have the fold down armrest with storage inside. They are interchangeable. You can put an armrest with storage where there was an armrest without storage.
There is no cutting, shaping or anything needed. They came like this from the factory.