As my prior comment - I think the manual advice is for a Beta engine S chassis - not an Alfa I chassis - Each type is a different width - Not sure if all the S type chassis are the same width - Craig may be able to confirm?

You are near right with your math - But an 8" wheel is 9" wide so you need to watch for that when calculating and the rim is getting close to the arch

There is some adjustability with the rod ends and a little bit of extra negative camber adjustment can pull the top of wheels in if it gets really close. Just don't overdo it.

You have a nice set of coffin spokes and the GP4 square arch is the best looking Stratos so I do not see a downside myself. You may find some 6J 5x112 steels from an Audi or a Passat with a silly offset if you hunt for some . A set of space savers may give that Ice rally event vibe but not sure the speed rating will work