Charlie,
In retrospect I too doubt the Campi's are any different. More likely I suspect aftermarket wheels for a GT5 and GT5-S are different backspacing to the tune of the spacer that was fitted to GT5 models to push the wheel outward.
Numbers for width I found varied a little, but suggested the GT5 was wider by a mere 2-3mm. The 5S numbers were more consistent, so maybe I should just measure my GT5 for comparison.
Good luck with the restoration, I have an area on the floor pan just like yours...... a weak point. I have other projects on the go and don't want to touch the GT5 for the exact reason you show....it escalates.
Julian
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