Rear deep lip on first gen F-body (???)

[LSfan70s]

I've been searching a lot but with no succes. So feel free to answer my question with links (PM me if need) or hit me with key words to help me find what I'm looking for.
What I need is the specific: parts, measurements, brands, work to do, or whatever I need to get 18"x12" with a nice deep lip in the back of my 68 F-body (Firebird for what you can see in my sig)
Where I live there are no experts or people that has done this before, but there are very good experienced welders and body shops that can do anything.
Basicly, I guess what I need to know is the housing length I need, how high and wide I must mofidy the wheel wels, do I need to modify the rear chassis to get 12" wide wheels?
I know there are plany of people that has done this before, but how? Or who should I call? Or what magic measurement tool should I buy that will let me know all this info.

Thanks guys.