Those two tires virtually identical diameters. Are you wanting the larger diameter 18" wheel?
The 45mm offset it going to make the front tires 68.5" wide which I think might be a problem, and the 41mm is worse. The 54mm offset makes it 67.79" which I think is a better fit for a 255 tire. I don't know how your frame is designed behind the k-member but I think that combo would fit in my frames. When you're pushing the limits like this, the only way to tell for sure is to try them. I recommend doing it without springs, turning the wheels lock to lock, and moving the suspension all the way up to the top of it's travel.American racing makes the torque thrust wheel I like in a 17x9 with 45mm offset/6.77 backspace that is the C4 fitment. To go with 18's the custom shop can make an 18x8 with up to 41mm offset, or a 18x9 with up to 54mm offset.
The good thing about that scenario is you can always add a spacer.The inboard clearance is my biggest concern at this point, but I also want less positive offset so the wheel has a little deeper dish to it.