in addition to the comment about not being able to slide sideways and crushing your spine in a roll over, theres one other concern that need to voice about the way you have your belts mounted.
it's incredibly dangerous to mount the back of the belt to the floor behind the seat. it's meant to be mounted perpendicular to the seat, presumably on a roll bar.
let's examine the forces placed on your body if the truck stops abruptly. naturally, your body is going to want to keep moving forward and smash your head into the windshield. but with your belts, your shoulders will catch on the belt and because the back of the belt is mounted to the floor it will cause your spine to compress, which is not a good thing.
if you mount the back of the belt perpendicularly, then the force it directly in line with your motion and will stop you in the safest way.
these belts are not safe for stree use because of the roll over danger that was already mentions. nor are they safe for off road use until you change the mounting point of the back of the belt.
check out these sites on the proper mounting of a 5-point. the same rules apply for a 4 point.
this one explicitly shows how you need to mount the back far away from the seat:
http://www.soloracer.com/harnschrothfaq.html
this one shows a picture of a properly installed harness
http://www.ntpog.org/reviews/safetyharness/harness.shtml