I have and 1940 knuckle with S&S 80 inch jugs on it only one hole on the head was pluged and redrilled. Does anyone know if pan or shovel jugs are a closer fit?

 

Knuckleheads have a different bolt pattern than pans and shovels. To use a pan barrel you will have to use a couple of stroker plates to make up the difference in height. The Easy Rider Tips and Tricks Vol 1 has an article on fitting a pan jug to a knuckle. If your heads are already plugged and drilled determine if the S&S barrel was a pan or Shovel. Use a head gasket to tell.