STOP! Don't weld on those mags! The slots are for multiple bolt circles. I promise you that if you just get the right mag lug nut (depth and thread size) and a mag nut washer it will work fine. I've used a plethora of those type wheels back in the day and they fit everything! Besides, if those wheels are actually Magnesium (which they may be) I guarantee you don't want to get them hot enough to catch fire....if they do, DO NOT TRY TO PUT OUT THE FIRE WITH WATER! The oxygen in the water makes a Magnesium fire even worse. Smother it instead. And if they are cast aluminum (which I suspect they are) then they won't weld worth a tinkers hoo-rah any way. As far as the lug nuts are concerned, the tapered shaft is just to facilitate starting the nut through the mag wheel. I strongly suspect that a straight shouldered one will work for you. Most passenger car and light trucks from the 40's on up have a 1/2"-24 thread. Hope that helps.
Rob Hawes-Anchorage Rod Works-Anchorage, Alaska