Toni...have you measured a Sub vs your Sierra's length? Looks like you have the short bed and that is about the same as a Sub of that year. Look at where the rear passenger door edge is to the bumper vs my Sub pictured below...about the same
Remember, a half ton and 3/4 ton Sub are the exact same length. Just taller for the 3/4 ton because of the higher rated stuff below the body sheetmetal...
Fleet service has a 3500 Suburban and they are hitting the used market now
My K3500 Suburban (tongue n cheek...cuz the 6K RGAWR is the exact same axle as the dually of that year {upgraded the MC & cylinders to the 1 ton dually}...and AAM rates it at 10,000 GAWR...ordered the F60 option, which begets the snow plow prep and 1 ton front springs)

Click For Full-Size Imagemysub mysuburban