Jast,
You first need to find a left and right frame horns, I had the same issue, the PO on my parts truck cut the frame horns off to make it a bumper-less truck (it was some FUGLY). I bought the parts from a club member you will need to measure someone's truck to get the exact length. Then I had a local welder weld them up to the frame. Check this guy.
http://4-5-6chevytrucks.com/