I have a 1998 mustang, v6 not sure if that matters. I want to put in sub boxes they will be right behind the seats. Is it ok to screw them to the floor there, or will I hit the gas tank or something else.

Is there a better way to mount the sub boxes?

 

You're better off to crawl under the car & check the spot you want to drill into,, gotta watch out for EVERYTHING like wiring harnesses etc.

Personally I just let my sub box sit on the trunk floor,, I don't bolt it down