You can. You don't need to because a 245/40/18 will fit on an 8" wide rim, but the 9" rim will fit. Remember the section width on a 245 is just barely under 10", which is what 9" wide rim will measure from outside lip to outside lip. The valiant's have a higher wheel opening in the quarter than the Dart's do, so it's a little less of an issue unless your car is slammed on the ground for ride height. On the 70+ Dart's there's about 11" from the quarter to the springs with the stock spring location. The 67-69 Darts are a little tighter, but not a ton and there are guys that have run 245's on them with the stock spring locations. You would have to measure your backspacing perfectly because you'd need your spring clearance to be at about 3/8" and your quarter clearance to be about 5/8", but if you did that successfully it shouldn't rub. With the tolerances getting that tight it might not work for everyone though, anybody that wanted to do it would absolutely have to measure their own car.
I would say it's more headache than it's worth since you'll still be stuck with a 245 and running the 8" wide rim will squeeze the 245's in a little to make life easier for clearance.