Frame Stiffing ERW vs DOM

[pontiacmcl]

Looking for some input. I am working on an A body (GTO) and am wanting to stiffen it up. I am currently welding the seams up and will be boxing them in. I am looking at doing some thing like what HPI customs (and others ) have done and weld some tubing under the floor board and add a cross member. I am by no means an engineer, so I have some probably basic questions. I for this intended purpose should I go DOM or is ERW more than enough (about 5,000 difference in tensil strength 55000 vs 60000) I am looking at 1 1/2 .125 erw (1018) or .120 dom (1020) . I also have a design question. Here is what I am thinking.

The front to rear tube is straight in this picture with a brace from under the frame in the front up to it. I thought this would add strength to this weak area. Then I might step up to 1.750 tube for the cross members.



Thanks in advance.
David