Kind of hard to see just how it fits behind the shoulder with all that wool covering your mammoth
Unclearthur is your Thorowgood guy, but I'll give a shot at your question about whether you need the wider plate.
Are you asking because you have so much wither-clearance? I don't think the exact number of fingers clearance is that important, as long as there's clearance. Your pommel is nicely lower than your cantle. You look correct sitting in it. With a wider plate, the pommel will sit lower, which I'm afraid could really pitch you forward. Is there any bridging? If you have bridging, you might need the wider plate. If you don't, then things are sitting about where they should be. Can you see his shoulder-angle under all that wool? Can you lift the flap and find the tree-points and get a good visual on how the tree-angle lines up with his shoulder-angle? With the girth tight, have you run your hand under the tree points to see if you feel any pinching? If the tree-angle matches his shoulder-angle and there's no pinching, and no bridging, then you probably have the right gullet-plate in there.
It looks good on you!