dirt... I have a 41 Willys with a Pro Street frame. I made a bracket that I welded to the left side of the center section and made and bracket that welds to the bottom of the right side frame. The bar is about 16 c to c and rides level. The rear suspension only moves maybe 3 inches total when under way so it doesn't wiggle the hips at all.
My circle burner guys all say it is all wrong...should be higher or lower depending on who you talk to.
As for me... it's just above center and level and it works great. BTW I use one urathane rod end and one steel to keep the rattle down..