Just changed the oil... and some issues....?

I drained and filled my 99 V6 firebird with 10w30 GTX oil. The manual says 4.5 qts. So i filled it up until half a qt was left in the 5 qt container. Started it up, and now the oil pressure is about 80 psi. Way higher than normal. Also, when it's warmed up, the oil is wayy higher than full on the dipstick. Why? I added 4.5 qts just like it said to.