Tim,
I think your plan is workable. I would be concerned that 8' is a long way between spans and might be prone to sagging and snow loads might become a problem. 2x12x16 rafters will eventually sag under there own weight but not enough to be a real issue.
Having said all that. Consider a product that I use all the time here in the Dallas area and available through Leows, Home Depot or your local lumber yard. Rafters that are engineered from plywoods. They are basically I beams consisting of a 7/16 web and 2x2 caps. Significantly stronger than 2x12's and much lighter.
As big trees are getting harder to come by they are sometimes less expensive as well. Might be a cost effective alternative.
By the way, did you make it to Oshkosh. I didn't fly Grezdlitn up this year but did drive up to appease the wife and look at some things in between hear and there. I was most impressed by Glacier Girl. I'd been waiting to take a look at her.
Mark
