I did it on my 70 Challenger so I could get 6" BS. Very happy with the results, 9" rims still have 1" clearance to the lip.
One thing I was disappointed with was the good Dr didn't mention you will have to "trim" quite a bit of the rear valance bracing for clearance.
When I ask about this afterwards, the response was, "you don't have to trim the standard valance". So if you have a dual exhaut valance expect to trim.
Not really a major issue but kinda of bugged me it wasn't mentioned originally.