From a strictly aesthetic standpoint...
...it looks very bad/awful.
Even with a lot of gravel you're talking 300 lbs. max. so it probably won't crush the heater in any case, but you could always climb up on top of it with a friend and see how it holds up. If you must, at least cut a couple of 2" x6's to fit under the tank lengthwise to support the whole base of the tank and distribute the weight more evenly - less stress to the tank, and it will look better as well. Better yet would be to just build a frame around the heater, though it seems a good way to cook some fish even if you leave clearance as heat rises.
I recommend that you start thinking about plan B.