If it's really just the thread coming out you can unbolt it from the
car and re sew it.  But I wonder if the thread rotted out and that the
seat belt webbing is also rotten.  In an accident it will just tear if
that's the case.

There are some plastic inserts that turn about 180 degrees in order to
remove them and then it's still a pain to get them out.  Once removed you
will have access to the bolt that holds the seat belt to the floor.  You may
need a long breaker bar (or length of pipe over a rachet but if it's that
tight you might break your rachet).  And you'll need "Star" drivers to
remove both the plastic and the bolt (if I remember correctly).

With something like a seat belt I prefer not to do any repairs that "might"
work ;-)  If it's too difficult to sew then bolts might be an option but I'd
get some rather large bolts and make sure they have lock washers and
possibly glue the nuts on with epoxy or whatever.  Places like Harbor
Freight (I think) sell stitching awls that will penetrate 1/4" leather or
rubber.  They have a wood handle and a spool of waxed thread in the body.
You push the threaded needle through the material, back it off a little,
slip the thread through the loop, and pull it out.  Then make the next hole.
Tedius, but it works and will be strong if you sew all the way around where
it was originally sewn.

Like Ashton said maybe you can just resew it or find one at a junk yard.