another overheating problem

Got the head back. .009 warped. So that is fine he didn't tell me to take out the stud bolts for the exhaust and intake manifold. Charged me another 48 cause the bolts were a ***** to get out I guess.
No I need to check the block. Can I really use plate glass as a straight edge. If so I was gonna pick up a piece at home depot or Lowe's. Gonna stop at AutoZone and getter a smaller feeler.
Head is nice and clean other than a bit of carbon left in the combustion chambers.