Kurt,
GregTag is our resident cheerleader of Ithaca guns. He tried to organize us into a recognizable group a couple years ago. I don't know if he was successful with this ragtag group. I'm now officially an enthusiast of Ithaca guns with a Crass, a NID, plus 2 Lefever Nitro's (in 410) which were an Ithaca design I believe. And, of course, we have Walt Snyder here, who is the greatest resource of Ithaca information and author of the Ithaca book.

Given that your gun is a Flues and a 'twist steel' barreled one to boot, I'd recommend low pressure loads in that gun. There are many commercially available and of course, reloading recipes are endless for low pressure loads.