Dad had an Astro with the 4.3, and IIRC, it was 18, maybe 20mpg. It was supposed to have 3.42 limited slip--but when the axle seals were replaced, it was clearly an open diff, so it may have had 3.08's for all I know. Anyhow, the point was, a V8 swap would/could only make better mpg's if you were to ignore some of the emission equipment (mostly exhaust stuff) and run a nice steep gear like a 3.08, along with EFI and probably just a 305.

After all, it's just a big box on wheels, and not a light one at that. I've heard some people comment that the full size vans would get about the same mpg but with more cargo room etc.