Best I can recall....the 4.3 and the 5.7 GM V8 share the same motor mounts and much of the geometry. I suppose you say a 4.3 out of a 93 Astro because that's what you have.
My suggestion? Leave the 4.3 in the astro....sell the astro, take the money and buy a 350 to drop in there (and buy a conversion kit) OR...replace your existing 4 banger.
You are going to have to fab motor mounts, relocate your radiator, re plumb your exhaust etc. etc... Getting your transmission right is even going to be a pain. Then you need to look at axles and if they can handle the additional HP of the chevy 6. Oh yeah... did I talk about springs? Power steering?