Funny enough, I just did this repair, literally today. The factory bolts are about 1.25" long and M8x1.25 just a FYI. Anyway i broke the exact same bolt, drivers seat rear bolt that sticks out of the bottom of the jeep. After it broke i tried heat, pb blaster, oil of wintergreen ect and couldn't get it out(grabbing it from the underside of the truck and trying to screw it towards the ground. Anyway as for what i did to remove it...
Took a carbide burr on a straight die grinder and ground down what was sticking up on the interior down to flush with the floor. On the outside I also used the bur and then finished up with a 4" angle grinder to grind that flush. at that point what remained of the bolt literally fell out. I then drilled out the hole to about .400". After that I bought an M10x1.5 bolt, nut, and two wide washers. I installed the bolt from the BOTTOM, so the threads were sticking up on the inside of the truck. This will prevent the threads from rusting, and allow easy removal in the future. I put a washer under the head of the bolt as well as underneath the nut inside the truck. Worked like a champ, though I will say I am going to put some rtv around the the head of the bolt/washer, so as no water/salt ect gets inside the truck. since the repair is not as 'water tight' as it was before it broke.
The only bolt out of the 3 that are the same on the seats, that needs to be as long as it is, is the one that sticks outside of the truck. The ones on the front can literally be like 1/3 the length they are and still work fine, b/c it goes into the seat bracket then like 1/16" of metal, and then it just floats in space. :yelclap: