No real need for the bell to be that thick, especially the rear.

You could easily lose 2-3mm there and get them re-anodised or whatever coating they have

But as the arch is fecked now anyway.....may as well just do all the work to the arch to accommodate the wheels/tyres, and perhaps a size or 2 larger.

If you were to try the likes of 888's on it, they tend to be wider than most tyres of same numerical size. Worth baring in mind.

if you can give yourself a good 5-10mm clearance over what you have now, you'd be well covered for the future.