The original frame is probably the best option. Why can't you use it? The S10 swap is problematic in the execution of the details such as running board mounting and bumper mounting as it has been related to me by someone currently doing one.
I have heard of someone using a 78-83 GM G-body frame (cutlass, Grand Prix, Monte Carlo, Regal) but like anything "custom" it will not come with instructions as someone once told me.
We may be able to provide further comment if we knew what your build plans for the truck were.
