In some ways I think the Caravan might be a little easier than the Astro.

With the Astro, it mostly bolts in but you're constrained by the close-fit OEM structure, and to R&R the engine in a van ain't easy.

But to put an engine in the middle of the cargo floor (probably have to put it right behind the front seats in order to get a driveshaft in there), in a wide-open area, thru the side door, sounds easier. If there is an extended-wheelbase model that might be the best start to get good driveline angles.

Yes, you'll have lots of engine-cradle and axle mounting fab to do, but it's a fairly blank slate without a lot of interference from existing structure. Run some 2x3 or 2x4 box tubing longitudinally and then tie front and rear framerails to it. A 4-link might be the way to go for rear suspension? A Ford Explorer 8.8 might be narrow enough.

Radiator stays up front and plumbed with extension pipes, use elec fans to cool it.

Fuel tank needs a re-lo? I presume it is currently forward of the rear axle?

The existing steering and susp would be OK, just pull the halfshafts out. You could even leave the original engine/trans in there for ballast if needed, depending on how much the mid-engine settles out. If you need less ballast, remove the engine/trans, and you have a nice hole to locate suitable ballast.

I’m thinkin’ a smallblock underneath a sturdy metal shroud, with some good insulation on top, some fake cargo covering it, would make a helluva sleeper. This is a neat idea!

And hey, if you plan it right, you might even still be able to drive it around some while you're working on it!