ijust pulled an r34 apart, and theres quite a few places you could get the petrol fumes, check it's more from the front or back of the car like in the engine bay ( injectors, fuel lines)
if you can smell it fairly strongly in the boot , i would then suggest it's coming from the fuel tank, one area it would leak fairly easily is the fuel lines out the top or connectors as suggested above, but there is also a big seal on the tank were all your fuel lines go in, if this is not positioned correctly fumes would be definately leaking into the cabin, this normally upset if somebody has fitted a hi flow intank fuel pump or repaired the fuel gauge sender etc.
cheers russ