I usually unbolt the metal tabs off the factory radio, then use them on the Metra kit with the mini carriage bolts supplied. You can use the flush plate if you modify the rear support in the dash.
Im glad you posted this !! I couldnt figure out how the Metra kit went together. I got the deluxe kit with all the brackets and adapters. I couldnt find anything that lined up with the four holes in the dash. I never knew I needed the two brackets that were on the Delco radio ! A friend of mine hooked me up with one of his friends that owns a stereo shop this morning. He told me what I needed, went to the local yard and grabbed them out of a Cutlass.
Now I just have to figure out which face plate to use. I think the flat one might be the choice.