I'm 6' and fit comfortably. The newer cars have a dropped footbox which adds a little room.
As for construction: The FFR can be built using a donor car or almost completey with aftermarket parts.
My small block car was built going the donor route, and with the BB I pulled out all the stops and the only thing mustang is the steering shaft and the clutch and brake pedals. As a result, the cars look similar, but they are two totaly different animals.
Anyone can build an FFR, but as is the case with many things, some people are better in certain areas than others. Everyone gets to the point where there's something they get hung up on, and if you do, there is the forum Bluovlrcr mentioned, or there's a whole list of people who specialize in building FFR's that can take over.