306heads

Never heard of 306 heads. There is a closed chamber small port 302 head that came on 318's in the mid/late 80's. This head came with I believe a 1.78 intake, not sure of the exhaust.

And, there is a 308 head which is an open chamber head with the improved exhaust port of the magnum head that came on 360's in the lat 80's. The 308 head because of the improved exhaust port is better head than a J head. The 308 has 1.88 intake and 1.60 exahust valves. J heads have the same exhaust but could be either a 2.02 or 1.88 intake.