pitsen3, You would probably want to set up the mounting for the 350 and fabricate motor mounts to accept the 4.3L. I am wondering that since there are some Chevy's that are normally equipped with a V8 but can be ordered with a 4.3, would the mounts for that vehicle serve your needs!? Some Caprices and Chevy trucks came with V8's or V6's so I think they may have a set up that you would need. Not certain though. The 4.3 and 350 have the same block except the 4.3 is about 4" shorter which changes position of the mounts.
I am about to put a small block in my Astro which came with a 4.3 and had to fabricate mounts to reach 4" foreward to accomodate the V8. Everything else is pretty much a bolt in.
Hope this helps. Brian