I probably did well enough on the PSAT to qualify. I live in an ACT state so that is the test I have prepped and taken. Do I need to take the SAT or will my ACT score suffice?
Yes, you will need to take the SAT to become a National Merit Finalist. Here's a quote from the 2016 requirements for the competition:
"To become a Finalist, you must confi rm your PSAT/NMSQT performance with scores on the SAT. It is solely your responsibility to:
Take the SAT on a date authorized by NMSC; and
Request that the College Board SAT Program send an offi cial report of your SAT scores to the National Merit Scholarship Corporation (code 0085)."
You can still focus more on the ACT as your main standardized test, but if you really want the National Merit honor you must take the SAT at some point as well.