The rust free ones certainly chew up[ a fair few rust repair panels don't they but looks good so far.
Think long and hard about bubbles. Its your car and Im not telling you what to do but with an unmolested rear 1/4 it might be a shame to cut all that good original steel out. I have a bubble arched Mk1 race car. If it wasn't needing much bigger wheels much further out I wouldn't have arched it, I was never happy with the fiber arches either and have just received new steel ones to be welded on. Mine also had rust in the arches meaning I was already cutting away steel.
Just a thought. What do you intend doing with it? will it be harder to register arched?
But best do it now while its apart than a year later.
Nice shed too. and don't wait 3 years to update us again
JP