I bought a whole 48' flatbed full of tandem cutoffs at auction for an average of $700 each. I had no use for them, but single axle cutoffs went for $300 each. That was at the West Monroe sale. I expect the Dallas sale to go for the same price.

http://berryhillauctioneers.com/auction.php?aid=561&page=inv

I have so many air ride suspensions that I haul them off for scrap. If you can find an axle with the ratio you like, you can use any air ride on it. And the frame width is standard 34".

But you will find if you put faster gears behind a 6 cylinder, you won't have enough power to pull it.