Something to bear in mind is that you will never get even arch gaps on a 1/200 series car (without it looking really stupid), as the rear arches sit lower at the back - have a look at your side-on photo, and note the difference in the gap between the top of the arch and the belt line. The rear will always look more 'tucked' at the back if the car is sitting level.
The only potential solution to this which I've seen is on the Vox 242, where front arches were fitted to the rear quarters, although in that case they were still fitted lower down the body, albeit the difference was made more limited by the front and rear arches at least then having the same profile. I think a better solution is just to embrace the look, though.
I think the different offset is a red herring - it will need to be different because the rim is so much wider than the factory-fitted options. The same thing applies to wheels with a lot of dish.
What you may run into here is simply that putting 18s on a 240 (and maintaining decent suspension travel, and making all fit and drive normally) isn't trivial - it's a 60s design, essentially, and the arches aren't huge, etc. You're slightly better off with a 7/900, but even they have significant limitations unless you start hacking at them - much above 225-wide rubber and fitment gets "interesting".
cheers
James
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