I traded a Monte Carlo SS for my GTO, but the trunk was "almost" a deal breaker. I hate beyond words that fuel tank, but as stated...it has to be there in the good ole US of A. But there is one way to put a box in and have spare tire access. -- And sorry, I have no pics of this...as it is a work in progress. But I've had it fitted up and it will work...mine's just not permanent (yet).
Go to a hardware store and get some long threaded rod (at least 3/8's) Pull the carpeting off and use the holes already in the cage for the plastic snap buttons already as a guide.
Hold your empty sub box up to it and figure out where you could run the threaded rod thru your sub box. Dril thru the cage holes at the areas where you can bolt thru the corners of your sub. I think 2 at top and one in between speakers will work.
If you'll back up to say 8" subs, and build as low of a box as possible, You can suspend the box off the rear cage area by drilling some holes and using the threaded rod to go thru your box. Then install nuts & washers accordingly on the box and cage to get it all sitting like you want it. The paint all the nuts\washers black and reinstall the cage carpet (with the rods holding it. This will hold your box 'suspended' of the tire area....and I think with 8's you just can wrestle the tire out.
There might also be a way to mount the spare tire underneath the car under the big black plastic cover that is where the fuel tank would have been on a Monaro.
I am currently working on hanging my box off the cage via threaded rod, and will have it finished by years end. Will post pics if anyone interested.