This is a common situation that I never get a good answer. Here is my take.
The code says a firewall has to be "self-supporting" (IBC 2003, 705.2). AS you can see the code commentary uses a typical wood-frame townhouse application. These systems have been designed, tested, etc for this application. When you try to use a "shaftwall" for anything other than this condition, it becomes problematic. For example, the break-away clips on each side of the wall are designed for wood framing and not existing buildings. In addition, this wall is typically 2HR, not 3HR.
So what to do. For a 3 HR wall, cmu is probably the most suitable for your condition. Since the existing building is 2 or 3 story, that can be done with cmu, probably 12" with some pilasters. The question we have is how is this wall braced or how long is it supposed to stand after one side collapses? For example, assuming this wall goes 3 stories + 30" abvoe the lowest building, you have a wall about 40'+/- tall. Building this to be free-standing, although possible, is probably not practical for most projects. Therefore, you (or at least the engineer) will want to brace this wall against something, probably the new building since that is easier. As such, how is the connection designed? Per the code, it should be either designed as a "break away" connection similar to the shaftwall or if supported by the building will need to have the equivalent fire-rating. If supported by the building, you now how to rate the entire structure for the same rating as the wall. This sounds good, but if you have a 7-story building and it collapses, I can assure you this wall is coming down not matter what the connection does.
Then you say, use a break away clip. That has other problems. The shaftwall system has a standard engineered, tested, etc off the shelf clip that you can specify. The cmu wall has not such system (that I am aware) therefore you and the engineer have to design, engineer some method. Then you have to convince a code offical this system works even though it has not been tested or approved by anyone but you and the engineer. Good luck.
I know I have not really answered your question. I would avoid the use of a shaft system for this application. The main problem you have is the height of the wall. It will likely exceed the avaiable stud gage since it has to go 40'+/-.
I would go with cmu that is somewhat free-standing and braced with a break away connection to the new building. This will increase the 1B to 1A for at least the portions that connect directly to the wall. Some argue the entire building would need to 1A since where do you drawn the line of what is supporting what. My gut tells me to rate the 1B to 1A to atleast one story above the wall and leave the remaing stories at 1B. I have no backup for this statement.
It seems like this is a condition you would find in Boston or NYC all the time. I do wonder what they do there.
Hope this helps.
Thanks for your thoughtful reply.
Big Surprise, this project was put on hold shortly after that post.