i dont think it's a bad idea to get it off the floor, though im not sure about the length you can extend an sata cable.
maybe try isolating the hard drive from the solid mounts first. Also try mounting it vertically with the plates spinning on the axis that you're turning. (i.e. stand it on end with the plates facing the front and rear of the car) this way the only forces that would make them hit each other would be acceleration and stopping... rapid acceleration and rapid stopping.... dont hit anyone and you should be good. im one of those that had a 3.5" mounted solidly w/o a problem... so im just brainstorming here