it sounds greaqt in thoery, but at what size do you start sacrificing stability. imagine all the metal you would have to use to keep things from warping and twisting, you might as well build the whole thing out of billit aluminum and make it fire m80's and walk on its own
most are going to be a metric machine panhead, their are places that sell common sizes for pretty cheap online, or you can go to the local hardware store
edit some are going to be high low thread for tapping into plastic