These are a common swap or "build with" with the Factory Five bunch. The diff carrier is aluminum but has a single side rear end. A 8.8 Traction Lock carrier can be used, but the side gears are different in the IRS rear end in order to accept the axle retaining clips. The side gears from the iron case T-Bird Traction Loc can be swapped in (or use the entire T-Bird 8.8" Traction Loc diff) or the original side gears machined to hold the clips.
The other 'snag' is the 5 x 4.25” wheel pattern. Obviously this can be redrilled to whatever pattern is needed at some additional cost.
But for those wanting an IRS, these are a good way to get there. The calipers and rotors are said to be good units as well.