I don't think that's a very good idea, because you are essentially left with a kluged together bike at great expense. Or at least if an f-builder has any self respect that modification is going to cost you. One thing to consider is if the rear triangle doesn't fit big tires, neither does the fork. So you are radically changing geometry if you put another fork in the frame.
I say sell and start over. If a custom isn't in the cards, there are options for this kind of bike that will work just fine.