heads question

A 1969 340 should have X heads. But the J heads can be improved with a good porting job and 2.02 intake valves. They can be safely be made to flow 245 to 250 cfm on the intake at. .550 valve lift and about 165 on the exhaust at .550 valve lift. That is only about a 10% to 15% improvement over stock but some of the aluminum heads don't flow much better out pf the box. So the improved J heads should work great in a street 340. Not sure about hardened valve seats though...