who sells mandrel frame rails?

get them from autoweldchassis.com, you just give them a list of your measurements according to their drawing, and they send em out, you can get 2 full rails for under 1000 bucks....

That seems kinda Spensive.... I only need the back half, under the cab is just gonna be straight anyways, and the front clip will stay stock, Just went to the A.R.T site and they say they are around $250 for a pair of 2x3" .120 wall

that 3x2 .120 wall is meant for a roll cage or at least a roll bar...... i would recommend the 4x2 if you are just doing a simple backhalf...... if you intend on using the 3x2, you are going to want to brace it up well with some triangulation....