looking to find specs on 340-X heads.

Fishy, I believe the 'X' heads (894 castings) were only used on the '68 and '69 340's. The 'J' (915 castings) were used on the '70 thru '73 340's and the 360's of the same years 1970-1973. c
Please heck this though, I could be wrong.
Terry