Just about all of the aftermarket Y-block cams I've seen are single-pattern. However, I know Ted has used dual-pattern cams on some of his builds. If the exhaust flows less than 80% of the intake, should a dual-pattern Cam be used. Just looking at some of John Mummert's numbers, it appears that the exhaust flow of G heads is about 70% of the intake. I have Sanderson headers/w 1.5-inch primaries and 2 1/4-inch dual pipes/w flow-through mufflers. 

As a generalized rule, I will use single pattern camshaft on a racing application where the headers are being run ‘open’. I do prefer dual pattern camshafts on street driven vehicles where a full exhaust system is being used.  There’s enough back pressure being generated by the exhaust system to require a longer exhaust duration (more degrees) than what you see on the intake side in which to compensate for.

Lorena, Texas (South of Waco)