Thanks all. It was a real struggle from start to finish but really satisfying. I've done very kind of construction trade you can name, designed and built fine furniture, etc....you name it....but, hands down, this was the hardest project I've ever undertaken. I think so much of that was the anxiety of knowing what the stakes were if you managed to screw it up.
The alcove originally was home to a gas insert, which offended me every time I looked at it. One morning, while the wife was at the grocery store, I couldn't take it any longer and ripped the whole sucka out and threw it in the yard. Yeah, that was the easy part. I built out the backing with Wonderboard on a steel frame made from rails used to mount electrical service boxes. I got real lucky on the tile on the backing as they spaced out exactly without having to cut any.
My luck ran out when I was bringing in that 7/4 limestone hearth, bumped it a little too hard and it split in two pieces....but, I bought another one and the two others are now table tops on a couple of steel pedestal bases on the other side of the room.
My luck ran out again when I realized that the class A run would have to come within 1 1/2" of a roof rafter. Yeah, just a 1/2" shy of the rated clearance, but I just wouldn't have been able to stop worrying if I didn't do it right. So, that was a long day on my back in the attic, wiping sawdust out of my eyes, cutting and boxing the rafters and all that goes with that. It was sort of like building a house in a phonebooth, you know? (Oh, and the next day I went to the ER and was diagnosed with both strep throat and pneumonia, yikes!)
After getting it done but for the surround trim, the wife and I lived with it a year before deciding what it should be. We ultimately agreed that it should be the same travertine, only in a smaller dimension. More vertical tile work, my least favorite kind. The Gods finally smiled on me. On the horizontal dimension on the top of the surround it came out dead-on without me having to cut any of the 1" tiles. Really, I was stunned. You don't get that lucky very often, and I'll take it!
The last bit to finish was to put two metal registers high up on the sides of the alcove (not visible in the photo). That allows heat to rise up inside the chase and circulate. It really has made quite a difference in the BTU's, more than I would have imagined.
Somehow, all the struggles are forgotten on a cold night when the room is toasty, the kids are lounging on the hearth and the spousal unit might be forced to admit that maybe I do know what I'm doing....some of the time. Thanks to all on this board who have provided me with knowledge to do this job safely. It is much appreciated.