Welcome to the forum.
The manuals at the Heritage Center are available online.
https://www.gmheritagecenter.com/gm-...tion-kits.html
That shows the options that were available when the truck was built. GM in the states wasn't great about keeping records for the old vehicles though. And over time people love swapping engines, transmissions, and axles so you should plan on doing detective work to learn what is actually installed in your truck.
Do you have any pictures of the truck? If you plan to do a significant amount of work to it you might want to start a "build thread" to share your progress.