Jose... I looked at that page and I have to tell you I am not the one to ask about that stuff. You are talking about doing mods to your car that I know next to nothing about.
*edit* I didn't look closely enough at the page the first time.. there is no picture of the kit you mentioned but at the bottom sure enough it says "New 9" Ford IRS is a complete assembly from crossmember to gear lube. It comes complete and ready to install in any daily driver, street rod, truck or all out show car."
That would be kinda cool, but like Chip said, is it really necessary?? It seems to make sense to just use the stock differential if you are going to stay with independant rear suspension. Sound Performance and SW have both run in the 9's with the stock rear end and I believe when the MVP Motorsports car ran in the 8's they were on the stock rear end too. Just replace the differential when it craps out.. probably a cheaper alternative. Then again, I don't know what you have planned.