First thing, I would advise against eliminating the lower chain guide. Not only does it insure sprocket engagement with a properly tensioned chain (which is seemingly quite loose), but it also serves as a sharkfin in keeping hands and feet away from a pinch point. OEM chain guides are usually one of the first things to swap out on an MX bike, especially ones made of aluminum that will end your day or strand you when hit. TM Designworks is the universal gold standard for replacement and wear like iron and are slick as snot.
As far as the upper chain guide, that is more supermoto specific. Not sure what year your EXC is but some people have added the SMR upper guide ($8) from the KTM catalog. Google Image search will give you some ideas and examples.
https://www.google.com/search?q=supermoto+chain+guide&espv=2&biw=1107&bih=590&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjFtMbK28_NAhVBXWMKHV2WAlkQ_AUIBygCMy set-up never required me to "push" the chain away from the tire and stray from the actual chain line. The upper chain is under tension during acceleration and may strain a guide that was out of line. Much of the chain rub occurs under decel when the lower chain is tight and the slack has moved to the top... Older, worn chains have more lateral play and O-ring chains tend to be wider, both exaggerating the issue.
Another factor is that each tire has a different "actual" size, apart from the "label" size. This clip was taken from thumpertalk:
I have a 5inch supermoto rear rim and have slight chain clearance problems with a conti road attack 160/60 tyre on my DR-Zed. As every tyre manufacturer seems to interpret 160mm diferrently... I found this list in a ktm forum, and added my tyres actual width to it. this will be a usefull resource for anyone with similar issues. feel free to add to the list..
5" rim
michelin pilot power 160/60 is 169 mm wide
bridgestone battlax rain 165/62 is 160 mm wide
pirelli supercorsa 160/60 is 166 mm wide
Avon pro extreme 160/60 is 169mm wide
Avon Distanzia 160/60 is 167mm wide
Koncorde 160/60 is 163mm wide
Dunlop KR108 165/55 slick is 157mm wide
rennsports 160/60 is 161mm
maxiss road legal slick 160/60 is 158mm
Conti SM 160/60 is 158mm
Conti road attack 160/60 is 164mm wide
Pirelli Diablo 160/60 is 162mm
The list was compiled in 2011 and includes the 165 Dunlop KR108 that I ran, showing that it is actually 157mm where as another manufacturers 160 measures at 169mm. Looks like you've got some homework to do...
One thing to consider if you ever dish a wheel away from the chain is the exhaust can. Even with zero dish, many of us need to space the exhaust away from the tire but it usually rubs a bit anyways on big compressions.
If you haven't already checked, supermotojunkie.com and thumpertalk.com should have more specific details regarding your bike, just be willing to sift through the nonsense. And I hope that my posts have been helpful, I would hate to be contributing to the nonsense.
