First of all I don't want one mainly because it will raise my center of gravity and they don't hold much fuel (want to beable to drive the cars as if there normal and have them normal looking). As far as the fuel tank goes I would like to just lower the stock one a bit and keep it like that.
What rules would make me have to have one?
Would I have to have one when I hit a certain speed in the 1/4 mile?
The Z28 will have nitrous and I don't want the 2 tanks back there, I don't like it.
That's for the Z28, the street machine.
For the V6 would I need one if I wanted to run diffferent types of road corses or some thing like that?
The only events that I know you have to have them is the drifting D1 class or what ever they are and some circle track events, I know there has to be a lot more.

 

This depends entirely on that type of racing you are doing. In most types of racing you have to be going quite fast before needing a fuel cell. But each oganization is different. You're going to have to check the rules, which are usually available on line.
As far as size and center of gravity go, you are mistaken. I currently have a 30gal. fuel safe cell in my 84 Camaro, and they come bigger. I have it mounted in a "NASCAR" style frame which lowers the CG from stock.