John,
I just went through this same process about 2 months back. I had some telling me do 4130 and others DOM and some shops out there are just using mild steel. When I tried to research rules it seemed like there was no standard. Also it seemed like I read if your floor and firewall are modified or not original that could push into another class. It was all over my head and I think Chicane's advice is good. I am a nobody when it comes to this stuff but his points are things I discovered. The whole post heat treat kinda scared me. When I started my car it was just gonna be a cruiser and by most points still will be just that however the street/track car is appealing more to me. Perhaps in my case starting with a heavier A-Body was not the desired vehicle but it is what is. I also hope to be at the Silver State Classic next fall (Maybe be two years) or Pony Express, Gamblers Run etc. It appears that (I could be wrong) if you show up with a car that has a cage that does not look like a backyard job you will be OK in the lower classes. I believe there are many production cars running in the lower classes. In the SSC the lowest class is 95 MPH avg over the 90 miles you cover. In my case with no driving credentials the max class a rookie can run is 110 MPH avg with a tech speed of 124 MPH.....still sounds like a blast to me. Return trips will with a good staying in the rules record will likely yield an elevated class. I like you don't expect to win anything and in reality my car will only see open track days on road coarse and hopefully some auto x. So in the end I also elected to with 1-3/4" 120 wall DOM. I still have to ad a few things to cage but am happy with the way it came out. You can see pics here
http://ls3-chevelle.com/Current_Updates.php , And here is some more reading if you still are pondering 4130. A friend put me in contact with John Parsons and John suggested this link for some reading
http://www.netwelding.com/Welding%204130.htm
Good luck on the project and I like the gumballs I got mine in the mail the other day. And if the stars align for SSC maybe will see you out there.