I've been wanting to do this to my truck and done some research on it, the best I can find, you might have to move a crossmember, or modify it for the tank to fit, it will fit between the rails and in front of the axle as far as I can tell. It would be best if you could remove it from the X yourself so you can take pics and document exactly how its mounted, and the differences between an X and your truck, also take any and all related parts to make mounting in your truck easier.
If you do this please, please, please, take lots of pics, and give us a good write up on it, I WILL be doing this to my truck eventually.
Do you mind sending me a PM, and telling me where you found the tank for that price, I looked for a month last summer and found one used one, and they wanted $245 for it, with no skidplates or anything. If you decide not to do it, please let me know, and I'll buy that tank from you.