Frame mods, hold onto your hats... Here's the way I like it, look at the profile of your frame before you start, then look at it afterwards, if the basic frame looks the same then you did fine. Widening for fat tires, as long as you are not fitting something stupid in there then it's fine, all the rear stay widening I've seen has been so negligible that it most people wouldn't have even noticed it. On my first build off bike I cut the seat tube top so that the clamp would be flat against the top tube, some people cut it completely off and use a wedge style seat clamp, doesn't bother me, frame still looks the same. I also cut a little from the top and bottom of my head tube because the fork I was using had a shorter steer tube, I could've modded my steer tube, but it was just easier to cut and if I hadn't said anything about it, nobody would have noticed. Some people have used dropout extensions to put their wheel further back and make more tire room, but once again, frame looks the same.