For the time being, I ran the amp ground to a paint-stripped/cleaned-up/dialectic-greased, front left of the four bolts that hold down the parking brake assembly.

It may not be optimal, but it was convenient and seems to be working fine. I had as many installers telling me that a body ground was fine, as I had installers insisting that a frame ground was necessary. I didn't feel like drilling a hole through the floor metal to affix the ground to the frame. I'm capable of doing it, I just didn't feel like it, and I used the conflicting opinions to support my complacency. All installers I spoke with, though, seemed to agree that the ground wire (4 gauge, in my case), should be kept as short as possible; no more than 9"-12".