You need an extra hole, no way around it. I opted to drill mine through the seat brace in the floor, since it is heavily reinforced, and used large washers so it wouldn't pull through.
There are multiple threads on this already... I searched and found the one with photos and so here:
http://67-72chevytrucks.com/vboard/s...ill+hole+brace
I'm not an expert, not an engineer, and I doubt this would pass DOT regs.... emulate at your own risk!