Need information on 340 J heads

All letter heads flow within +-5% of each other and all can be had with 2.02's either by opening up their integral seats or by dropping new 2.02 hardened seats in. Purists will like the X's but they all are the same flow characteristics, even the T/A heads. T/A heads can be MADE to have larger intake ports by the offset pushrod hole, but they are the same casting, just final machined differently. 308's flow better out of the junkyard with a smaller valve with better port velocity.