Mount them as tucked up into the floor board as practical. I have experimented with adding a short length of exhaust pipe after the cutout to direct the exit where I want it.
I also found that adding an oval shaped megaphone to the exit tip greatly sweetens the sound, much like the end of horned instruments.
1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 3.73s
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3600 stall lock-up converter and 3.91's (closest thing to an automatic 5 speed going)