actually, I think John's suggestion may be best, unless you are going to go ahead and use a fatbike drivetrain and 170 or 190mm rear hub. Because otherwise the chainline is going to be a problem. I guess there is no reason not to use a fatbike drivetrain on a road bike, but I'm not sure it's worth the hassle. I know one thing for sure, before I built a bike like that I would make sure the resulting q factor works better for the rider.

you can get different spindle lengths for Speedplay pedals.