I looked into this a couple of years ago and found a couple of places that may be able to help with an off-the-shelf axle shaft.
Try emailing these two people:
Kevin Yutani -
cvo@cvaxles.com
and/or
Ricky Grimes -
technical@rockfordcv.com
Both of the people were very helpful and had some very inexpensive solutions to narrowing up the T-bird IRS axles.
TWM