I think that a slightly tapered worm could lead to the same result. It's possible also to press radialy the worm onto the wheel as this is made on many rotary tables. An other solution is to use a wheel made with two parts joined on the throat plane.
A friend of mine have created (and sells)a software for this purpose, It is efficient for helical internal gears as the tool price is important and the helix angle small.
the load on bearings increases with the pressure angle, for quiet running use a plastic gear (the biggest one)if this is possible (check for stresses).
