It seems to me that the only thing you need clarifying is the width of the chassis. You already know that you have a 1.8 engined 4/4 !
The cross head on your 4/4 will be narrower than the Plus 4 but of course that is at the wrong end of the car for your measurement for the panhard rod! I'm not sure exactly when the chassis became a standard one for all models but I do know that Budster and I spent sometime measuring our respective cars and realising a cockpit width difference between his 1.8 4/4 and my 2009 Roadster.
It may well be that your car was built at that crossing point.
