Filtration could be an issue with setting the tank on the floor. Many canister filters rely on the sipon effect to feed them. With the filter sitting pretty-much level to the tank, you may have some problems getting things to circulate properly. i would say get a smaller tank and do it right, over doing a half-hearted job on a large tank.
Edit: also, running a gravel vac is difficult. When we were moving from our apartment to our house, i had my 25g on the floor for a couple weeks. It was super-dificult to get good suction going with a gravel-vac.