Do you have a rollcage? If not, you will be safer without a harness or seatbelt. If you do not have a rollcage, I would suggest a helmet, and safety clothes, by that I mean clothes that will give you some degree of protection in the case of an accident.
A full face helmet is a must!!! Roll bar must go over your head and be able to withstand the force of the kart rolling at top speed without folding up.
You would realy be suprised what can tip when it goes sideways at 30+ mph. I have seen racing karts roll several times before coming to a stop, most of the time the driver is lucky enough to walk away because he/she was thrown out.
You want this thing strong enough to keep you alive in a worst case scenario at 20mph over your predicted top speed. Look around at some pro designed karts with cages.
Pretty much every car will slide before it flips, VERY difficult to flip most cars unless you hit a curb or something, or you have some crappy 4x4 with suspension from the 50's (*cough*jeep*cough*)
I also think you should get the seat first before making the cage, that way you know exactly how much room you have.