340 X head advice

Only concern with the X-heads is that you don't know how many valve jobs have been done on them since 1968. The more valve jobs a head has, the further down they sink the valves. The further the valve seats are "sunk", the worse the flow. I don't have specifics, but it would be possible for a 1.88" 360 head with new, 2.02 valves on fresh,"high" seats to outperform an X-head with multiple valve jobs - all else being equal.

There's always the LA-X or EQ closed chamber head option. Unported, these were over 2 tenths better in the 1/4 mile when compared to 1.88" LA heads with the same prep. (1.88" vs 1.92" and chamber size was about 5cc less on the LA-X.)

Have you talked to Tim Sloan in Sanford, NC? He has a lot of small block experience. He builds a lot of NHRA/IHRA class legal engines as well.