... or maybe a shorter prop-nut that has a center-thread for screwing spinner cones on. You'll have to measure the depth of the spinner cone, and compare that to the length of your crank/prop-nut/prop when that's assembled. That should tell you if that prop-nut assembly is too long. Occasionally these can be shortened, just with a Dremel cut-off wheel, as well as the spinner-screw that sometimes needs to be trimmed, so it's tight when the spinner cone is firmly seated too. Don't tighten that spinner cone too much though, you can warp that backplate by tightening the cone on too tight. Firm is good for a cone, but extreme tight isn't good for the cone or backplate.
Maybe some pics too might help.. I'm only imagining... but a 1/4" discrepancy/space seems quite a bit.. especially if they told you it would work and it's the shortest nut they have.