It's been a while since I've grokked my R2 '67, but I'll bet the parts common to ANY version of a '67 (or future '68) are grouped, and parts unique to the FB are isolated.
A body is a cored tool - the expensive part of the date - so that would have some cost implications. But now with new methods and suppliers, maybe it is not the hill to climb we'd once imagined.
I think whoever DID bring it out, if it had the build friendliness and detail level of a Moby Ford pickup would be raking in the Benjamin's.
Can't conceive of the retailer or distributor who would see a well done package, and not instruct the buyer to 'load the wagon' with product. They would literally just lift the page from Dearborn, Metuchen or San Jose, and make the kits a "have it your way" bonanza.
But that's just me and I could be wrong about that.