I bought a new thermostat housing , thermostat and gasket for my s10 with the 350 in it .and i noticed the thermostat housing has a o ring in the bottom of it does this replace the old paper gasket or should i use the paper gasket ? the old thermostat housing didnt have the o ring in the bottom of if,… dunno what to do lol sound funny but ive never seen this before…
You should be able to use just the o-ring, it’s a better design overall than the flat.
Don’t go solely by me 
I say this because I’ve seen GM use either one on the same engine.
don’t forget to bleed after your done too
o-ring
just make sure they didn’t staple the o-ring as that has happened to me