I wouldnt use the saddle without a gullet. I think it would weaken the structure, and I also believe it would cause it to sit low and awkwardly on the wither.
And if the saddle is still too narrow even with the largest gullet fitted, I doubt it would be wide enough without a gullet anyway. There is a reason that you can only fit so wide a gullet, and that reason is that it just wont bend out any wider.
FYI, if your dressage saddle is not an Isabell, then it cant be the same model as an Isabell. Each model is exactly that, its own model.