the 245/40/20's would look good. you can see them on ATP's shop truck here...
http://67-72chevytrucks.com/vboard/s...d.php?t=343061 as far as backspacing etc. I would take the truck to discount and have one of the managers measure the backspace for your truck, and take the guesswork out of the picture. the 5 x5 pattern is the 5 x 127mm so they will fit for sure. however i would have them measure and order, then test fit, any issues they correct for you.. good luck.