Sting,
Muriatic acid (liquid pH decreaser) is recommended for people on well water with very high alk readings. It's a quick & easy way to lower alk when instructed how to do so properly. As a result of using this, you will knock out pH but it's always critical to adjust alk first. The pH will tend to stabilize once alk is in the correct range. 80 -120 is goodand being that you're not too far off, diluting granular acid (pH decreaser) will do the trick when the correct amounts are added. This is based off of your volume and initial alk readings.
This adjustment isn't immediate and you don't want to over-shoot the correct reading. It might take 24 -48 hours with steady filtration to get the results you're after. Be patient. For this reason, I don't recommend using muriatic acid in your case.
Steve