You may be able to collapse the "shell" of the remaining bolt & threads inwards with a sharp punch & hammer...but those seat belt bolts are very hard steel,so it may resist your efforts to do so...maybe you can use a thin metal cutting blade in a sabre saw to cut a slot in the remnants of the bolt,that will make collapsing it inward a lot easier..
Trying to use a tap now will likely result in a busted tap and much aggravation...if you have a cutting torch its often possible to heat the remains of the threads up red hot and blow it out with a touch of the oxogen trigger..then use a tap carefully to re-cut the threads..or you could just put a nut on the bottom instead..