At most I'll use the car for an occasional informal track day or auto-X. This is just intended to be something more substantial than the convertible top bows in case the car did roll over. Also, it will give me a proper place to anchor some shoulder harnesses.

I have a removable hard top for the car and it's possible that no event would let me in without it being on. Although I seriously doubt it would give much additional protection. I may even be required to have a full cage for either type of event. In that case I would most likely do something bolt in, since I don't want a full cage for daily street cruising.

Since you have experience building race chassis, would 1 5/8" 4130 be enough for that? The car should weigh 2500-2800 lbs. Most as built small block cars go closer to 2500. I'm running an all aluminum engine, but I have added some additional steel to the chassis.