Excellent responses men! You guys never let me down.
So, the build continues tomorrow by adding the durarock on top of the bricks and attached to the wall standoffs.
I will implement both of your suggestions by using hardibacker, not durarock, for framing and then late when we fire it up keeping an eye on the hot spots. This means we won't be putting the rough sawn lumber on for a few weeks so we can do our testing, but, we're ok with that. Patience will pay off here.
One other question about the length of the exhaust tubes. I was going to go with the 30 ft. length for 8" pipe, but, if you look you'll see we have the burn chamber now on the right side in the photo. This means my length off pipe may now only be 16-18 feet. Too short?
My concern is that I need to put the external exhaust pipe venting out behind the barrel. If I include a 6ft vertical before exiting the wall I could now have 22-24 feet before going outside.
I suppose I could do some rather innovative permutations in the bench, but, do you think it's necessary?
Ray