My opinion is that the BXA is overkill for the 12x36. I had a 12x36 and replaced the stock lantern-style tool post with an AXA and it fit nicely and worked beautifully. The tool holders are slightly cheaper too.
There are or have been two or three people on this forum who had put a BXA onto an Atlas 10" or 12" (bed length doesn't enter into it). And a similar number on the Atlas/Craftsman Group on Yahoo. But as I've written two or three times before, the list of people who have put a BXA QCTP onto an Atlas seems to fall into two groups - those who've done it and say they regret it, and those who've done it and won't admit that they wish that they hadn't.
I also have an atlas 12x36, and purchased an AXA holder. I did a ton of research into if I should get the BXA, I liked the idea of more capacity, but really, the 1/2" capacity has been more than enough. And getting in tight areas has still been difficult compared to using my rocker post, I can only see a BXA being even worse, the AXA is also right about in the center of its adjustment with a 1/2" tool on this lathe. I don't have any regrets going AXA. Nor do I have any 3/4" tooling to put in a BXA. 1/2"HSS bits work great, as does my 7/pc indexable 1/2" tooling I picked up from grizzly. Hope this helps, just a little advice from someone that was in your same position.
Sorry, but less money spent on the BXA would still be money poorly spent It's like putting a 2HP motor onto an Atlas simply because you found one that was much cheaper than the 3/4 HP that should be the max size..
The standard AXA #1 and #2 holders were intended for 3/8" square tooling and will hold 1/2". The #2 will hold up to 5/8" diameter round boring bars. The AXA #1XL and #2XL holders will hold 5/8" square and the #2XL will hold 3/4" round.