Swapping over stuff to get the shell rolling is easy but do remember things like front hubs, rear anti rollbar and gearbox cross member are different on a 3 door although sapphire parts will fit. If you look at how it all fits on the one car take some pics if needed then unbolt it all and swap to the other shell. Is the 3 door shell a cosworth one as there are some differences if it is not. But nothing that can't be worked around