Mine is in a plastic pod at the bottom of the dash, though I've also seen them fitted in the pod to the right of the steering column. Because it's so far away and dangerous to adjust while driving, my radio (an old Sony off the car boot sale for £2) is fitted with a wired remote control that I've stuck to the left of the steering column shroud.
This is the mount I have:
http://www.halfords.com/webapp/wcs/stor ... yId_165626
This is for a DIN-E fit stereo, as Mike said the Viva would have had a twin-post mounting radio rather than the later DIN fitting that was introduced in the 1980s, you could probably adapt this pod to mount one of those if you want that kind of originality, or find one somewhere. Halfords used to do them, maybe another audio place or autojumble still does. I had one somewhere when I used to have my Pye "Searchmachine" in the Firenza, but no idea where it is now.