Take this from a crusty old rodder.....the rust you can see is only 10% of what's there.
Make it easy on yourself and begin with a Southwestern car.
Take this from a crusty old tech inspector....
Have a shop build your cage that does a bunch of NHRA cages as their specialty, preferable close to an NHRA chassis certification inspector who can go to the shop and tag your car before you pick it up. You would not believe the number of cars I have had to trailer because of illegal cages.