An Astro with 6 lug wheels? I think you might be thinking of the Chevy Uplander, which replaced the Astro. It has 6 lug wheels, but is a front driver, not rear/all wheel drive like the Astro. I'd be somewhat leery of using an Astro for a swap like that, the Astro doesn't really have a chassis so to speak, it's built on a unit body typw platform, there's just a front sub frame as far as engine/trans/front axle goes. Also, the engine is offset to the right by about 4" to cleat the steering shaft, the center cection of the rear diff is offset the same amount. They also have issues with idler arm wear, which is double on them because they actually have 2 of them.
Inmy own opinion, if you want to go 4x4, find a mid '70s Chevy 4x4, get the axles and other 4x4 related stuff from them, and mount them to your stock chassis. You'll get a more original looking,and better performing for off road use, truck. Try to find either a '73-'76 or if a later 1/2 ton, one with a 4.10 axle ratio. The reason is that those trucks have the stronger Dana/Spicer 44 front axle, and up to '76 will have a 12 bolt out back. Aslo, look for manual hubs on the later truck. Most of the manual hub trucks have the NP205 cast iron T-case, very strong. Or alternatively, a '74 ('73 on Blazer) -'79 with the NP203 full time case would be exellent, too, but fuel mileage may suffer.
