Someone correct me if I am wrong here but the gpi doesn't determine the spine of the arrow. Just because they are heavier per inch does not make them stiffer. A .340 is stiffer than .400 and .400 is stiffer than .500 regardless of weight. I would recommend going to Easton's shaft selector to make sure you get the right fit.

http://www.archeryoutdoors.com/shaft...afts/step1.tpl