Tom's running a 285/35/18 on an 18x10 up front, but yeah, he pulled the fenders out a bit.
I have 275/35/18's on my Duster on 18x9's. My Duster is being cloned over to a Demon, so it has Dart front fenders on it now with no issues after I lengthened the stock fender-to-bumper braces a smidge to push the lower fender corners out. My 18x9's have a 35mm offset, but I have a Dr Diff 13" brake kit which adds 3/16" to the offset in addition to the 73+ disk track width. I currently run 3mm spacers too, but I probably need to go to 5mm spacers as my tires are just touching the frame at full lock still. So if you add all that up its about 25mm of offset on 18x9's for the stock 73+ disk track width. It would be the same for 18x10's since offset doesn't change with rim width (backspace does).
The '76 Dart Sport that Hotchkis is building also has 275/35/18's on 18x10's.
I think 18x10 is the most you're going to run up front, and that will probably require tubular UCA's to clear the rims at full lock. And a 275/35/18 is probably the widest you can really expect go without starting to modify bodywork. Some cars can obviously take a little more, but that depends on how much wheel lip rolling and fender stretching you're willing to do.