Any car with a frame (like a '68 Chevelle) must have all tubes welded to the frame or frame members.

Go online to NHRA and buy a current rulebook ($10). There's way more than can be addressed here, but I'll give you an idea of a few things to look for with this article. I wrote this for a roll BAR, but most of it is applicable to a CAGE. Main difference is that you'll be using 1 5/8" tubing for a cage, whereas a bar must use 1 3/4". The main thing I would caution you about is checking wall thickness on every piece of tubing you're going to use before you ever strike an arc. There is an allowance on chromoly of about 10% due to manufacturing tolerances, but none on mild steel. It must be 0.118" minimum, PERIOD. Some of the builders in this area will use 0.125" DOM just to be sure.

http://streetmachinesoftablerock.com...opic.php?t=217

Here are some additional thoughts from an "interview" by Streets...
http://streetmachinesoftablerock.com...opic.php?t=174