2002 SL front seat mount

The front seat of my 2002 SL is kind of low for me, so I want to remove
two rear bolts that hold the seat in the place, replace them with longer
bolts, and have ~1.5" spacers between a car floor and seat mounts.
So armed with 8mm socket I tried to unscrew those bolts, but they were so
tight that I could only slightly move them about 1/16 of a turn, and then
there was a great resistance. I didn't want to break a bolt head nor damage
a thread, so I didn't apply a full force, and left it alone.
Did anyone do something like that? Any suggestions?
Thank you.

 

Since they're a critical part of the restraint system, I would
suspect that the bolts are somehow special. Certainly they are
extra-strong material, and you will need to replace them with ones
with a similar rating. And they may be lock-nutted or cottered or
some such to keep them from rattling loose.