If you are using standard seatbelt, you just bolt in 3 places. On the roll bar as in picture above. Then bolt on two places on the chassis. Holes are already there and tapped.
You will need a long seatbelts beleive it or not. Like these.
http://www.cbsonline.co.uk/other-seat-belts-213-c.asp
You only need the bar for harnesses.