Now you've gone and done it! You've given us enough info to criticise you on.
1. The class you selected, As I recall the record is over 250. I would be going after a "softer" record on my first try at Bonneville.
2. 72 Camaro/608= With that body/ engine combo, you'll be making more power than the body will aero.So you should expect to experience wheelspin before you get to 250, if not entire vehicle spin?
3. The rollcage= Only 120ft? Well that will be a good start. I prefer 1.625 (1 5/8) x .134 The smaller diameter bends tighter to give the driver more room inside. Don't be afraid to use extra bracing. And don't be afraid to cut the roof off to build your cage. I know that sounds CRAZY, but GOOD welds all the way around the tube are critical. And if you are not a good welder, hire one. Roll cages,,,I could go on and on,,,well on to number four.
4.Where are the pictures? We need pics so we can drool and cry. A little out of envy,,, or jealousy.
Sounds to be a really cool project/journey. Sounds like you're acquiring some really cool parts(jealous). And as always,, Make it safe, and try to have some fun doing it.
Good luck, RICK