I just put in a new pioneer unit. They fit right into the original opening no problem. The tight spot is between the lower side of the face plate of the new radio and the outer dash trim piece. To get around this, on the black plastic bezel plate that the radio slides into, I had to oval out its attaching holes in the steel dash a little, this allowed me to slide it up a little. With this change then the face plate of the radio will just barely fit above the outer trim piece.
For the rear side of the radio, you need to bend up an L shaped metal strap to support the rear of the radio to the lower edge of the dash.