The seatbacks can be modified for the belt to slip thru, but you'll need the trim piece for the front, and the bezel for the rear to make it look nice. Or you can swap the seatbacks....
On coupes, there will be a threaded boss on the side pillars for the upper belt slide, under the quarter trim piece. Convertibles of course will not have this...the pass-thru on the seatback is all there is to hold the belt up, afaik. 

Hi Adam, thanks for that, I'm guessing there must be some kind of a tunnel attached to the front or back trim piece for the webbing to pass through the seat back?

Just ordered belts for my convertible too. What may be a problem with running the shoulder harnesses through the back seat is that some of the buckles do not fit through the bezels in the upper seat unless they are detached and re-sewn. The ones I bought have a ring that is attached to the upper side of the seats. This might not be the case in your situation but I thought I'd mention it incase you were stoked about using the bezels as I was only to find out it wouldn't work.

Jon

Just fitted new lap belts as a temporary fix will look at a solution for lap/diagonal belts later I think I'm ready for test now then next step will be registration so I can finally get out to drive Graham