That was originally the plan for model year 1990. However, so many '89s were still sitting on the lots at the end of the '89 model year, they sidelined the plans to build them for 1990.
You have to keep in mind these were expensive trucks at the time. They stickered for nearly $17k. That doesn't sound like much today, but at the time I bought one of mine I could have bought a new extended cab Dakota for $11k.
The Shelby option was $3,933. There was quite a bit of engineering in these trucks. Shelby didn't simply take a V-6 truck and drop the V-8 into it. The trans crossmember was moved back. A new front crossmember was welded in for the V-8 engine mounts. A heavier radiator was used. Since there wasn't enough room for an engine driven fan, 2 electric fans were placed between the radiator and a/c condenser. This required special condenser mounts which spaced the condenser farther from the radiator support and closer to the grille, which in turn had spacers to give the condenser a bit more room. Also, the wiring for the fans was integrated with the computer and was fairly complicated. There were also Shelby specific exhaust manifolds, computer, and air cleaner. The radiator hoses had 'CS' part numbers on them. Plus they had all the Shelby specific interior and exterior parts.
Dallas