Hi Daniel
Finding focus won't be an issue. I don't think 2.7 inch wider is much of a problem. As it allows space for focuser baffles, filter slides etc. You can use a smaller cage which makes the size of all the components a little smaller which helps if space is an issue.
Try playing around with newt to see what works best:
https://stellafane.org/tm/newt-web/newt-web.html
BTW are you member of the ASV? if so why not come to the Instrument Making section meeting next Saturday afternoon. There are people there who could help you.
Rod