At least for SCTA events you have to meet the specs for the record in the class you enter. So if your car won't go over 100mph but you enter an class with a record of 250, you have to meet the 250 specs. Cage requirements don't change though a few other safety specs do when you go over 200. Cage specs are minimum required and generally people will build, or at least recommend, in this sort of case to over do it. Like go to 1.75" tube (or bigger) from the 1.625x.120 requirement. Cheaping out on safety specs is not considered wise......
. The addition of head restraint/full containment seats/"funny car" type driver enclosure etc add modifications to more standard cages. Be sure your cage builder understands. Also, moly specs are slightly different but as weight is your friend may not be considered beneficial to go lighter. I am not sure of any sort of official opinion but heard that for deformability reasons that mild steel was preferred but I am not sure that is true. Ask Steve Davies (chief car inspector or Mike LeFevers ( Dynoroom)who has already posted on this thread.
You are not but often people will try to repurpose a race car from other venues, SCCA, NHRA and so forth. Their cages generally will not meet SCTA specs so starting from scratch is best, as you are.
When you get your rule book you'll have a better idea about this.
There is a guy in SLC that I ran into last year at Speedweek. They were involved with a streamliner. His work looked good. You might try. Lockhart Enterprises. 801-485-8029 is number I have.