RPM Paul, I think you'd need at least a T5 from a 4.3L S-truck, and that truck's bellhousing/hydraulic setup, since the bolt pattern on the bellhousing will mate to an SBC. I know the F-body T5 has the tilted bellhousing, but that's so the trans mount can still be horizontal; I'm not sure if the truck T5 bolts straight-up in the front.
If the truck T5 mounts straight-up in front, you can bolt it to an old-school bellhousing and use mechanical linkage, but I think you want hydraulics in your case. Since you're adapting an SBC into a vehicle that didn't come with one, I'm sure you'll run into fitment/packaging issues with exhaust, and the hydraulic clutch setup is much easier to deal with in that aspect. Then you'll just need to figure out what clutch/flywheel assembly you'll need. For more swap info, check out
www.5speeds.com - hope we helped you out some!