To start, there are two basic kinds of 250 heads - integral, and manifold style. With the integral head ('76 and later?) the carb bolted directly onto the head, and these are generally considered worth their weight in cast iron (meaning - not worth anything). Very poor performance.
The non-integral head had a few variations, mostly changes to accomodate emissisions changes. Sorry, I don't have serial #'s but I do remember a post a month or two ago on this same topic - try searching the archives.
-Boyo
