special heads?

74dart said

The X head numbers are
2843675

and I have a rebuilt set of L heads I was told were high performance with the numbers being
4027593

They have some extra ports for water it seems like but the spring size, valve size and other ports all look the same as the x heads

Click to expand...

2843675 heads are not 340 X heads. I see the X in your picture but that's not where it's located on 340 X heads. The X on 340 X heads is located between the spark plugs as mentioned above and sometimes it's not even there but is on the underside of the intake runner.

There is only 1 casting # for 340 X heads and it is 2531894

2843675 heads are 318 heads used from 68-74

4027593 heads are 318 heads used from 77-83

Neither are high performance heads. Just standard small valve 318 heads.