On October 3, 1994, about 0753, eastern daylight time, N701AM, a Cessna C-421B, registered to and operated by Air Mobile Ministries, experienced an all-landing gears collapse on landing at Whitman Field, Stuart, Florida. The flight was a 14 CFR Part 91 personal flight. The planned destination was Port-au-Prince, Haiti. Visual meteorological conditions prevailed at the time and a VFR flight plan was filed. The airplane was substantially damaged and the pilot and six passengers reported no injuries. The flight was originating at the time of the accident. The pilot stated that after takeoff the landing gear would not come up so he placed the gear handle in the down position and observed three green lights. On landing roll the gear collapsed. The one passenger who was on board was an FAA employee in a duty travel status, from the San Juan, Puerto Rico, Flight Standards District Office. Examination of the landing gear safety switch [Squat] by FAA personnel revealed that the switch was corroded, full of water and was improperly rigged. FAA personnel also reported the airplane weight and balance data used by the pilot for takeoff computations was in error, and the airplane was about 200 pounds over gross weight on departure.