Goodnes...<g>
- I'll bet that if you could do it, it would cost at least 50% more (or greater) than a fully-assembled factory firearm -- gun parts (in every case I've ever seen) are like car parts...
- I'll also bet you probably can't get a receiver except as a factory-installed replacement (which means you'd have to send them an existing shotgun)
- I read someone state once that the federal tax issue you mention had an exemption for some very small number of guns (very small,but greater than one <g> ) -- check with the local ATF office, or maybe their webpage, but don't forget state laws
- Perhaps you could find a cheap [ab]used Mossy to send back and get the factory to put it in a new receiver of the type you want?
Good luck!