I know 13 inch alloys spell much more under arch room (and better acceleration).
I wouldn't attempt to fit the arch so that you are able to take the wheel off without first removing the arch, it's simpler to resign yourself to first having to take the arch off.
PM me if I can help with lengths/measurements but bear in mind mine is an early model and I think MNR have improved the design since.
Lastly, youve probably finished by now
Hope it went well!