Eric, I put a centre seat into my Ex-Mod three door 110. There were no mounting holes in the seat box, so I just centred the seat and drilled four. For the seat belt, I obviously wanted a shoulder belt, so I had a bar made up that runs along behind the back of the seats at head rest height. To this I was able to attach the shoulder belt and a third head rest. For the actual seat belt, I had two retractable belts that came out of an Ex-Mod 90. My 110 had mounting holes behind the seat box for three sets of belts so i just used the two unused ones. Popped out the plastic plugs and just threaded in new bolts. Forget the size but they are fine thread and I got them at Home Depot. I installed the centre belt so it clicks on the right, away from my belt, seeing as I'm the one using the truck solo most of the time.
I mounted the shoulder belt on the underside of the bar for now, but when my eight year old daughter gets a little taller, I can move the belt up on top of the bar to keep it in the correct position across her shoulder and torso.
Hope this helps.