My Jaguar has the gas tank behind the rear seat like your GTO and I never could get decent sounding bass in to the cabin from the trunk area so I gave up on that. It all started when the factory 6x9 "sub woofer" gave up the ghost as the foam surround crumbled. I ended up removing the stock sub, fabricated a new mounting ring and replaced it with a PowerBass 6x9 sub that does pretty decent for what it is. My car also has solid steel between the rear seat and the gas tank with a lot of sound insulation and I don't know if the GTO is like that or not?
I hope that in the end two 8" woofers IB works as I can't live without a trunk, need space for golf clubs. I think that if my Jag had two 6x9 subs it would be decent enough as I don't want to modify it to that degree either due to the nature of them getting harder to find in good condition.
I didn’t even think about the fuel pump noise. my Jaguar has two fuel pumps, it runs on one when there is no boost from the supercharger and the second one kicks in to up the fuel pressure and volume when the engine sees boost. I never hear them at all, but my fox body Mustang is a different story.