Although it might be legal below 500ft watch for military low level traffic. Get a map sowing their sreas if possible. At 420kts he will be only a speck on your windscreen then fill your windscreen. You will also be very slow moving in his eyes and it is often relative movement that triggers his view. Midairs in the past between fast low movers and light aircraft well documented. Even the wash from a 420kt guy can do you serious damage.