You may or may not have problems depending on the brand of tire you go with because as you know some run wider than others in the same size. Every guy I know that runs a 315 on an 18" wheels has some rubbing problems although not terrible, but some rubbing. Lot's of guys with 17" 03/04 Cobras are running 315's with no problems.
To keep about the same overall wheel diameter size, you have to go 315/30/18 and not 35.
I wish there was a BFG GForce KD in that size... they look so good!
A friend of mine has 305/35 on a set of 18" TSW Thrust. He has a 03 Mach 1 w/ Steeda springs.(about 1.5"-1.75" drop) The fender is cutting his tires up BAD. He has only put a couple thousand miles on them and they have some seriously deep cuts in them. Sorry, I can't remember what brand(I think BFG GForce KD), but I'll try to find out, and let you know.