I'm going to try to replace all the 6" pipe like Greg suggests, and leave the 6" collar for now. I have not measured the draft, but can do that anytime. I know from "feel" that it must be OK, but I guess I should live and die by the numbers. When I get to it, I'll post back. Guess I should do it before I change the pipe. But like Berlin says, I may have a good draft reading, just not enough volume.
My chimney is actually probably on the short side. If I make it taller, it will be very difficult to clean, as my roof pitch is 6/12. Right now, I think I can stand on the roof and sweep it. I might still be able to clean it if I add another 30" or 36" section (which I have laying around). As it is now, it's probably 10' long, but the top is about 18' off the ground.
And yes - codes and inspections are pretty lax in WY. But I'm comfortable with the old triple-wall pipe in my garage. The price was right.