My girl got me bolt on flares. long story short I feel bad cuz she really tried. so im putting them on...

needless to say truck is bone stock. I need to fill some gaps. it kinda looks like the train track truck lol. I thought spacers at first. but might as well get tires and wheels.
its an 03 6.0psd 4x4 sd lariat crew. I cant figure out that spacing. flares are wide, and i would like it to be flush. is it possible on 18" or 20" and bigger tire without lifting? they just look so small now.
any help would be great.