Depends on what you're after. Overall, they're pretty much same, and just about any year will do, although the later ones do have some manifold fitment issues depending on what heads they have. This especially comes into play with the Vortec heads, as the port locations and I beleive the manifold bolt patterns are different. There's also the issue of the rear main seal, whcih went to 1 peice in the mid '80s ('86 as I recall). You'll probably want to look for one from the late '80s to mid '90s, as they will be most adaptable to a carb, and will be more likely to not have so many miles on them. Late '90s-'01 (last year for the 350) wil have the Vortec heads, which would also be fine, but you'll ned to make sure that you get the proper manifold for it. Alternatively, you could use the engine as is, and convert to the EFI system. I beleive there is a kit to install a stand-alone computer to run it.
There's several kits out to convert the tank, but they are pricey. One option that I've sen to be popular, though, would be to use a '73-'91 Blazer tank. One from a Suburban might work, too. Or, another option, you might take a look at the mid '90s ford f150 auxilliary tank (GASP!), which mounts at the rear of the frame.
