Sean - Are those SB2.2 heads? And I wouldn't call a 500-600 shot excessive for a street car!If you plan on stepping up the nitrous tuneup on a motor making good horsepower already, like 500+, (which IMO is the only way it should be done) a stand alone system is definately the best and safest way to do it. Several of my buddies run like this and thats how we run the race car. For it we have a small tank up front and and it's a completely stand alone system. we run the same gas in it as the tank, 118. For tuning and changing tuneups it's the only way to fly (pun intended ).
We are running about a 500-600 horse tuneup in two stages, more than your average street car, but it shows how expandable and safe a stand alone syste m really is.
In the below pic you can see the tank, it feeds the upper regulators and the nitrous systems. The lower (red) regulator is from the fuel tank for motor only. It's obviously excessive for a street car (maybe ), but you get the idea.