GForce makes an extraordinary axle package for 8.8 IRS cars. They use 108mm Porsche CV joints and special high strength stub shafts for the center section. Provide the under car dimensions for your existing axles or send them to GForce and they will size the splined half shafts on the new axles for your car.
If you haven't already I would highly recommend upgrading to the 31 spline side gears or better yet Ford's 33 spline super 8.8 side gears and then use the corresponding stub shafts from GForce.
Here is a link to the product on their site =>
GForce Axles
Ed
p.s. I just noticed the SPF sequence number on your car. I suspect you may use other than a Ford 8.8 IRS. If you do use the 8.8 IRS , GForce is the answer. If you don't then you may be deeper into custom territory.