If you have an upright tank you probably want to put it to as close to the bulkhead as possible. Towards the front of the floor in the back there is like a secondary floor, which covers where the rear footwell would be if you had a corsa/crew cab combo. In the floor there should be little panels. If you unscrew these then you will be able to see where you are drilling, and you shouldn't have to worry about drilling into any mechanical parts!
I used the high tensile u bolts in the link above with the spreader plates.