Astro Vans ARE subframed vehicles... The whole front sub unbolts with six bolts and CAN be dropped out as described.
I have never done this to change an engine though. It is much easier to just pull them out the front. You won't get it out through the side door. The motor is to far forward in the van.
As far as using an Astro as a donor suspension? The hub to hub width on these dudes is 62.5 inches. Usually much to wide for even a truck.
The V8 conversion is pretty easy. It can be done with stock parts and makes for a real sleeper if you keep the exhaust quiet