OK, I got to looking at my notes inquiring about the chambered vs. transverse exhuast systems, and it looks like the in/out diameter of the chambered is 2". So, if Alan's exhuast is copied after the originals, it should probably be 2" as well.
Now, you mentioned looking for a 2.5" chambered system. The
only chambered exhuast that I know of that big in/out is for the late model '93-'99 F-bodys, offered by
Braxton .
The biggest problems with the original type exhuast is the usual bends and crimps in the head pipes and tail pipes, in both systems. So, I'm not sure how much difference their would be performance-wise between the two. Now, if you could swap out the crimped head/tail pipes with something with a mandrel bend in it, then that should yield a gain.