As wide as you can without creating a huge overhang. The numbers you gave, I'd think ~10" would be about as wide as I'd go- that leaves 52" or so between the inner bars, and depending on your boat and frame, I don't think that'd overhang too far for me.
My boat is 92" wide, frame is 80" wide, and there are ~52" between the inner rails. That makes for 12" wide decks, plus the 2" tubing. Pretty sweet to walk on. If you use sketchup, I could email you a file of the boat with frames. PM me.