Shade, go to partsnmore.com . They sell a very nice petcock (part no. 20-0100) for $30, and an adapter plate tapped for the petcock and pre-drilled for the 46 mm. mounting hole centers on your tank for $7 (part no. 20-0103). All you have to do is mount the plate, scribe the petcock hole, remove the plate, and do the needful by hand with a round file to open the hole for the petcock filter and stem (avoid using power tools that might throw a spark). I don't know if a gasket is supplied with the mounting plate, but if not, just get a sheet of gasket paper and an Exacto knife and cut one. I like Gasgacinch for the sealant, although the brush-on form of HK Copper Coat also does a good job.
I use a Pingel petcock, but only because a member here offered it for a low price, and it was the dual outlet type that I wanted to try as a way to clean up fuel line routing with a pair of TM36-68 carbs. Pingel's products are insanely overpriced.