You didn't say specifically, but odds are it's the bottom that's more wasted than the top/upper sides. You might look at getting a frame from a Ford or other car/truck, they are more common at swaps, on ebay, craigslist, fordbarn.com, etc. That way you could section in parts from the donor and wouldn't have the effort of fabbing a whold new piece. The profiles for all these are pretty similar.