DIN HU in old-school GMC trucks?

I bought a metra kit as for an 85 Suburban, which has a steel dash. The dash was designed for a shaft mount radio. I know I have to cut a rectangular hole in the dash for the DIN cage mount, but how are you supposed to secure it? The Metra kit appears useless insofar as securing the HU is concerned. All it has are some nice face plates and various plastic brackets which dont seem to be usefull( the kit is meant to cover 73-87 GM trucks, so it has a ton of stuff in it) any tips much appreciated!

There's a kit???? Cut it out..... build a backer out of abs for behind the steel, and to give the cage something to grab. Backstrap the deck. Can't even begin to fathom how many of those I've done... this is GM country...... The trickest install I ever did on one was a custom iso mount.... took forever, but that thing looked hot in there. Gus Wherever I go, that is where I end up......

Yes there is a kit.  Once you do the cut out, the plastic finishing piece is held in by screws, and you can mount your cage to that.  Works great.  Put some backstrap on it if you like.My ex once told me I have a perfect face for radio.