You can't get restricted fittings anymore unless you find one NOS, to my knowledge. You can take an inverted flare fitting, fill it with solder and redrill to the appropriate size. The original fittings were a compression fitting that had a conical tube nut that slipped onto the tube.
There should be a brass fitting both in the head and in the filter housing that will accept either a compression fitting or inverted flare, both with male pipe threads I believe.
I can't remember the size of the restriction, but it's got to be listed here somewhere, if not,
http://www.oldgmctrucks.com/ will have it listed on the site in one of the posts.
Jeff
