Our primary purpose it to apply the NH3, so 18" is fine.  We have put it on at as much as 36" in the past, and if you index to the rows and put the shank between the rows, there is no reason 60" wouldn't be adequate.  Wheat would be  a different deal. 

We welded the tubes straight to the points.  

Heck, we put them on half worn out think points which I expected to break before the season was over, but they lasted better then I expected.  As far as the fabrication goes, basically I bent a 90^o in the tube and welded them on, then bent them to conform to the back of the shank as close as possible once the points were installed.  Wasn't too much work involved in it.  I had an extra shank in the vice and stuck the point on it while fitting the tube, to make sure of clearances.  Didn't bend them very close until they were actually installed, so that they could be as close as possible without having to worry about clearing the socket for installation.