Get a piece of metal fabricated to correct the angle issue,attach it to the dash opening for the radio and slip your deck into this adapters measured to fit your decks opening.It will give you a clean look by not seeing the sides of the deck.All the other places you mentioned about possibly installing it will cause you the same grief with the angle.You could just mount it under the dash in front of your console as suggested by Brett.
David
http://FirstGenFirebird.org/show/closeup.mv?CarID=571 If i don't get this car back on the road soon i'm gonna go postal! On a quest for FGF knowledge 1968 Pontiac Firebird Convertible 1969 Oldsmobile Cutlass "S" Convertible *Sold*