Hi Keith,
The IRS from the T-Bird sounds good, but the Alpine is too narrow to allow an IRS to work well. The half shafts would be too short for the suspension to work well. One of my old customers sold his V6 Alpine and bought a Tiger that had had an IRS installed. I don’t know what it was out of, but he told me that the car had handling issues because of the limitations the narrow body put on the size of the half shafts. I suppose that if you were to install fender flares to allow the IRS to stay the stock size so it could work as designed, you could make one work, but since you said you didn’t want to alter the body of your Alpine, that pretty much does away with that idea. The T-Bird IRS was also designed for a much heavier car, so has springs that would be too stiff, and the car would ride like a buck board, hampering the effecient operation of the suspension.
A lot of ideas sound real interesting, but on further investigation, aren’t really practical. I don’t want to rain on your parade, just wanting to give you some information for you to think about. This is just my opinion.
Jose 😀