AFAIK virtually all the '68 Mustang FB stuff is available. You shouldn't have any trouble getting parts, ex USA via NPD or any of a number of suppliers.
But it's easy to get bogged down in a project. Just don't swap one seemingly bottomless pit for what could turn out to be another!
I agree, if your bored with the one you have, moving to another project possibly isn't the solution, if you want a falcon coupe, finish your mustang, if your not happy with it, then sell it and buy a falcon.
Others have said it, but parts and rust is a problem for these big coupes, replacement panels are not available and good second hand ones are not cheap.
I have a few falcon sedan parts I've been collecting, good front guards range from 250-500 each, bumpers expect to pay 300-500, grill expect to pay up to 1000 for a good one, a Gt bonnet again, up to 1500, things add up fast, your mustang I would think, would be 40% cheaper on parts, than a falcon.