Model A's are narrow. Model T's even more so. Most rodders use a rear end that is in the range of 55 to 56 inches wide at the wheel mounting flange on Model A's. A friend had a Jag from a XKE under his 29 roadster.
I suggest calling TCI, the frame people in California, and asking them about Jag rear ends and which ones they use. I would not start out with " I building my own frame and .........."
Jerry Kugel of Kugel Komponments is very knowledgeable about Jags.