[Simondm]

They have paid for the full area of the screen, so they'll use the full area of the screen.

Seriously, if it was me, I'd render-out a short section using various settings, to see what worked, and what didn't.

You can force the display, by adding your own borders left-right and rendering it as 16:9 (or 14:9), but IMHO it's probably better to leave it as 4:3 so it can be displayed to taste on the viewing device.

If you force 16:9 by adding your own borders, you can end up with the ridiculous situation that "Pick" Freeview channel often shows, of a 4.3 picture surrounded completely by a black border. I think it's because they just can't be bothered to check the content matches what they expect. So they set their 4:3 flag for old content, without noticing that the recorded format is 4:3 because it's already been pillarboxed by their content supplier, or on ingest.

With the above mangled output, there is no "zoom" setting override on our domestic TVs that will allow proper display without the border (i.e. with correct aspect-ratio pixels).

IMHO, 4:3 doesn't look too bad on a 14:9 or 16:9 display, especially bearing in mind the original CRTs were very often 5:4. A lot of the edges of the frame were cut off when originally viewed at home.