the cardboard really wouldnt show anythign since the fuel system should only be under pressure when runnign and then slowly lose pressure. Also, fuel dries very quickly and a leak may never drip all the way to the ground.
I remembered you had posted a pic of the engine so i went back and looked it over. I see a few things that id suggest you check.
heres the pic and i marked some areas....
Now, i'll first start with what i marked "B".
I see it is a returnless fuel line so that first makes me think whats going on with the factory return line. If that isnt terminated properly, fumes from the tank are escaping into the engine compartment or the drivers wheel well somewhere. That could be what you're smelling.
Now about C, those bunch of fittings could be leaking, it just seems those fittings are unneccessary. one continuous piece of line would work fine. The less connections, the less possible leaks.
As for D, just chekc that those fittings are tight and not leaking.
The main thing that concerns me is A... The factory fuel lines are hard plastic lines that run from the engine down into the wheelwell. In that pic, it seems like the factory line comes out of the well, and taps into the -AN fitting. as far as i know, there is no real way to connect the flimsy GM hard plastic line to an -AN fitting, thats not something thats recommended to do. I would seriously examine that connection and make some type of change asap if its setup like that. This is also the area where i'd look for the terminated fuel return line, it comes up with the main line, so you should be able to find it.
Now for my last thought.. there is no need what-so-ever for the adjustable fuel pressure regulator. I can see its still runnign the stock ls1 fuel rail with the stock fuel pressure regulator mounted on it. set up that way, you can set the aftermarket regulator at anything you wish and the rail will still only see a max of 58psi(stock setting), you could turn down the pressure but that isnt the goal. If it is an In-tank fuel pump system, its 99% sure that its for an lt1/ls1 application so whatever pressure it pumps out, can be handled by a stock GM regulator. IMO, and this is just me, i'd rip out that aftermarket fuel line and reg and try to find a line from an LS1 car that someone is parting out. take the line from the fuel filter to the engine and swap the whole line into your car, that way youll have a much easier time troubleshooting it IF the smell remains, along with that figure out a proper way to vent/terminate the factory return line, which is now probably left open and just acting as a vent for fumes.
