308 is open chamber, found on 360's later '80's up to '92. They used 1.88/1.60 valves. Exhaust side flows well, but the heads are prone to cracking between the seats. I've used a set, still have them, and they are cracked.
302 is a "closed" chamber 318 head used in late 80's-91. 1.78/1.50 valves Also known for cracking.
J heads really have no advantage over 360 heads, and even the smog heads will perform just fine, and port like no tomorrow. :D
