Julian your spot on, All the long door cars have the same chassis' and are the same dimensionally in the tub. The wing size differs in order to fit the wheel off-set dimensions and are individually cut to fit each car for precise fitting. This means between the different cars there is up to 3" difference in width with the 4-4 the narrowest and the roadster the widest, plus four in the middle. There has only been one anomaly in the long door dimensions and that was a plus 4 sport with a narrow front crosshead. It was built in limited numbers as a limited edition however some new customer orders were requested to be built like that as well at that time.
The Roadster as Julian said has a larger dimensioned gearbox and therefore the transmission tunnel is larger taking up more internal space in the footwells, if you wish to fit a footrest/ security foot locker in the footwell the Roadster needs the 10" version rather than the 12" example for the rest.
So in conclusion the later, long door cars are all the same width within the cabin area. As an aside fitting a foot locker is a very good addition to your car. It will achieve a secure place for valuables and a much greater level of comfort for your passenger. I am 5'10" and sitting in the co-pilots seat is quite a strange experience as your feet are straight out in front of you with nothing to brace you. This is especially noticeable under braking.
I think my 2003 runabout was a long door but on the narrower chassis....?
