You can use a set screw FSP and shave your existing FSP to fit under the MOE HG. I don't know if the shaven FSP will fit under it though. Do you already have the mid length MOE? What color HG were you going w/?
just checking. I had a OEM style middy in the parts box I would have sent you for the price of postage. It sounds like you have everything except the FPS. If you're looking to get it done fast you may want to get a set screw FSP until you can get the holes pinned for a PSP.