RE: Anyone here have a solid axle in thier Mark 8?
The IRS is a better setup. Unless you're planning on running REALLY fast (ie: gutted car, cage, $20k in the motor, etc.) don't bother switching. The only reason people with IRS cobras switch to solid axles is that the IRS in the mustang is a poorly designed piece. If you want the car to handle, stick with the IRS.
Oh, and no, it won't come anywhere close to "bolting up". You'd have to design your own subframe and brackets, there would be extensive cutting and welding involved, and in the end, you'd be lucky if it ended up as a safe setup.