Great info from Drudis, but that is a perfect example of the differences, and what to watch for. In straight line racing, the main hoop MUST NOT be more than 5" (don't quote me on that, just an example) from the driver's head, and the doorbars MUST pass the driver between the shoulder and the elbow.
Here's a pic of mine:
(More at:
http://knights-rule.com/Bill_stang.html)
For drag racing, I wasn't too concerned about the rear shock towers, but I definitely tied them to the frame. I also wanted to keep the back seat for those Saturday trips to Bruster's for ice cream.
Like Drudis said, do your homework, look around, make sure what you want/need. THEN start welding.