That is correct, the Astro is unit-body, no separate chassis. There would also be issues with is using the suspension and driveline components, besides the width issue, there is also an issue with the steering setup as well as the engine/rear axle being offset to the right by about 4".
If you're looking a going 4x4, the S10 would be a good start, and if you want AWD, the parts from the Bravada swap right in.
