3.5" should be fine. You didn't mention what front end you're using, how much narrower is it from the rear axle? You mention using spacers, that always concerns me, especially when they're used on the front. How much of a spacer are you going to use? You will want make sure the studs are long enough. Anything too much, you'll want the spacers that bolt on and have a second set of studs that the wheels bolt, as getting too long with the studs can cause stress to them. Also bear in mind, spacing the wheels out will alter the scrub radius as wheel as the wheel centerline on the bearings. You may experience some handling issues, nothing major, though, and you will want to pay attention to the wheel bearings for wear/play.
