The official record is Bruce Meade at 510 feet hit in 1978 in Amarillo, TX:
http://floridassa.com/Bruce.htm or
http://www.sandbagger.com/2002_hawaii_softball_dream_tour.htm
Second place goes at 508 feet to Mike Macenko:
http://www.angelfire.com/sd/slopitch/legends.html
If you're looking for the "unofficial" record it was said to be set by former Steele's player Rik Lucas at 535 feet. See info below....
http://www.rallytimesports.com/main.htm or http://www.wolverinesports.com/sfblvid2.html