That should do the trick for mounting the radio into the opening. The Retro Sound case is smaller (left/right) than a DIN so their case adapters work with their DIN adapter to accept the smaller Retro Sound case and hold it fast. You may have to mess with the tabs on the DIN adapter to accept the, presumably thicker, valence material, but gentle work with a Dremel and a cut-off wheel should fix that. You should also have a strap to a stud or something on the rear of the Retro Sound case to support it - the other end of the strap should either go to a cross brace (don't know what's behind an IM dash) or anything secure to hold the radio up.
You'll also have to measure the DIN hole that you have, and then call Retro Sound to make sure that the bezel they supply will fully cover your opening - It should (they seem to know what they're doing) but it never hurts to call and make sure before you take delivery, right?
That's a pretty good video, BTW... I have to practically take the whole dash apart to get my radio out. Maybe I put it in there too well? and I noticed a vibration buzz in it yesterday while driving.