"After 1987, R/V remained in use for the Rounded-Line one-ton crew cab pickups through 1991 (built at Janesville), chassis cab (which was phased out after 1989; single cabs were discontinued in the US/Canada where the production tooling was shipped to Mexico where it remained in production until 1991), and the Rounded-Line utilities (
Chevrolet K5 Blazer and
Suburban, built at Flint) through 1991. From the 1988 model year and onward, C/K was re-used for the fourth-generation "GMT400" design"
So there's some overlap in 3500 models between C&C, pickups (89 dually reg and ext cab are GMT400s pretty sure, but they also still made 1 ton crew cab squarebodys until 91).
Are you sure your's is a straight frame truck or just a pickup truck without a pickup bed?
Because if it's a 89 1 ton 4th gen, it shouldn't be a crew cab and it shouldn't be a C&C. But it could be a 1989 1 ton dually reg cab or ext cab pickup.