Glad to see your project is coming along.
As for the cage, most of the older trucks are running 1.50x .120 CREW mild steel. For new construction I would reccomend 1.750x.120 HREW for the maincage, and 1.500 for the rest. 4130 is too costly and not a good idea, DOM is okay, but spendy. A good brand of CREW is just fine.
Bolt in cage kits will require lots of additiional work to make safe and competion legal. You will need to add cross bars, diagonal bars, V bars, roof bars, the list goes on. Your best bet is to bend one up. If you do not have access to a bender (JD2, ProTools, ets), I am sure there are guys on this BBS that can do the bending. I do quite a bit of tube bending for people, and they take the tube home and assemble them. A 120vac welder is fine for up to 3/16. Anything bigger use a bigger welder or arc. Use lots of gussets on all the corners. Let me see if I can find some pics of mine 'unskinned' so you can see the tube work. Or attend some races and see what others have.
