I did much the same - at first the unit would not slide into the sleeve due to the bolt heads, so I Dremel'ed the heads down some. Then I didn't really like it overall, and brought it to work to get tack welded in place instead.

It's just a few weld beads in a few spots, so it didn't distort the metal any, and it's easy to take a Dremel again and slice right through them to get it totaly out of the car down the road.

When I was doing this, I first put my CD player into the bracket - the holes in it and the holes in my OE bracket matched right up. Next I put a couple of wooden shims on it and sat the Empeg on top of them and the CD player. I moved it around until the fronts of each piece lined up, and then had the tack welding done. When I got it home it fit great.

HTH, and makes sense.