I haven't been into the radio compartment of my '84 for a long time, but I remember there was a 90-degree bracket that screwed to the underside of the dash (using the four screws of the vent plate) which had a fitting to receive a bolt from the back of the radio. Do you have this bracket??  If not, maybe you can imitate it with a piece from the hardware store.

Ed
'84Westy
'89 Wolfsy