Welding X car questions

Hello Shopbot friends and experts....

Having gotten back home, I am now working on welding my X-car as Brady Watson recommended in my most recent post. I have found someone who will do the welding, but I still have other questions:

1) I have found a local supplier of rectangular tubing of the size that I need (2.5" x 1.5" with .188 walls). But it is available in two grades of steel, A513 and A500.

In hunting, I have found a description of A513 which says it has low weldability and low machinability. Of course since I have to have it welded, and I have to tap holes, I don't want A513.

HOWEVER, I can't find anything that gives a layman's description of A500. Is that good to use? If it isn't the right material, what material should I be looking for?

2) Being aware of post-tensioned and pre-tensioned concrete, I had wondered if while I am modifying my X-car, if I should put threaded rods in the lower area of each added steel tubing, which I could torque to strengthen the steel tubing, and further reduce sagging in the middle of my X-car (I have a 5HP spindle which is quite heavy). This seems like it would be easy to do, but is this over-kill?

3) Does anyone have pictures of their welded X-car? It may be useful to see what a well done welding job looks like.

Thanks, Chuck