Size of the pit lane - i.e. a roval or a 1960s-era road course? If you are dealing with 40-foot pit boxes rules don't need to be as strict as when dealing with Summit Point-sized boxes. In the former, staging equipment and crew is less dangerous than in the cramped confines of Summit Point.
What kind of barrier is there between the pitlane and the crew staging area? Is it a concrete wall (ala rovals/professional courses) that will keep burning fuel from getting into the staging area or is it a single strand of armco (see Summit Point) that would allow burning fuel to run into the area normally used for gridding cars (and where teams are staged for the enduro)?
We have managed to cram 70 cars into a pit lane that is very short and has very small pit boxes. And then with the same set of rules went and ran at places like daytona, Homestead ect with huge pit lanes and pit boxes.
As far as the barrier thing, whats your point? No matter WHAT your rules are people can still spill fuel and start a fire. Make fuel spills a BIG penalty, beyond that you can't do anything really.
You are trying to fix/make it hard for people to screw up. And you can't do that.
I have done more then 45 fuel stops with a regular old 5 gallon gas can never have spilt ANY fuel doing it. Yet dry break systems have been shown to fail and fires can start.