if its the same as the 79, the hole is the right height, but is too narrow by a couple of centimetres. It should have 2 partial brackets bent out of the dash 1/4 way down the sides of the holes and another one at the bottom of the hole, as well as a nut welded to the firewall as a securer for the radio. I found all the brackets to be next to useless, but by bending one side of the hole a bit larger, the radio fit very snugly, and does not move around at all, even without any mounting screws. normal size radios also just clear the firewall and happily sit on the heater assembly, so there is nothing to it if you can get it in the hole. by the look of things NA yotas got plastic s*** all over the dash which we didn't get till '80+, which may be getting in your way too?
sam