the method i used is to plug weld them from the back side
i drilled 1/4'' holes every 6-8'' in the bed sides (a little over kill),
clamped the rails on the bed sides and plug welded them thru the bed
if you want body color use plain old 1.25x1.25x1/8 angle and you can save a lot of $$$
i used regular angle iron, it made it easy to form around mini-tubs and looks good