First off . Ron You must love tinkering . ( not a bad thing at all ) but you seem to have gone to an awful lot of trouble and expense to save yourself about $15.00. Also the tread size on your mod 7 is 10 mm X 1.
While you are correct that making a petcock will undoubtedly be better than the cheap aftermarket ones I have seen. Your best bet may be to buy an original Puch Valve. I have the entire inventory of Original Puch valves For all Maxi, Magnums, and Cobra models. I also have some of the aftermarket ones that I use for paper wieghts.