This has been discussed before, so maybe someone should write it up properly and have the mods pin it, for future reference. The one and only snag, when considering this (in my opinion, VERY WORTHWHILE) modification is that we have good reason to believe that not all shells have provision for the locking catches. Later shells with rear seat belt mounts are ok, because the seat belt mount (item 63 or 64 in the link below) provides the window catch bracket too.
http://www.somerford...&id=1&chapter=5
However, earlier cars have two small brackets, item 15 in the link below, which are no longer available.
http://www.somerford...id=12&chapter=2
Frankly, the best way forward is, if needed, to get a full seat belt mount kit, item 65 in the first link, weld it in properly, and fit rear seat belts. That way, you get fixings for the window catches, and a valuable safety feature. AND, you never, ever again have to face up to trying to fit fixed side windows, a task that ranks somewhere alongside the brake and clutch clevis pins in terms of difficulty and sheer unpleasantness.
But there is a reasonable chance that your car already has the seat belt mounts fitted, or if older, the window catch brackets. Only an inspection of what is actually there can prove it, one way or the other. If you have belts, you already know that the answer is favourable. If not, and you can see the central belt anchorages in the boot, it is very likely that the parcel shelf/C pillar mounts are also fitted.