If you template is taken correctly, the shape tells me he's a medium wide at least - there's no way he's medium. If you know anyone with Wintec gullets you could borrow, they're identical in width to the Thorowgood and a blue one ought to fit.
Otherwise, your only real option is to trace your wither template onto a piece of card (with the straight lines added) then cut it out and take it to the tack store with you. Then you can offer it up to the fronts of individual saddles and check the tree points - you should be able to see the ends of most - run parallel to the cardboard cut-out.
If you did the same to the T18 (loins) you could offer that up to the rear panels, to check the gusset looks deep enough to sit the panel on the horse's back across its full width. And the back profile might help you see if the tree shape is about right, though that's a more difficult judgement.
It's not an ideal way to judge suitability, but we're trying to make the best of an awkward job :)