Difference in X heads & J heads

I did some search on the 915 J Heads int showed that they were only used in the 340 in 1970 is that right ?

I believe regular '70 340's were 894, but 915 was the casting used for 340-6 T/A heads. Just machined for 2.02 intakes and the offset pushrod holes. However, there were some very early 894 T/A heads which may not have made it into production cars. Another member here has photos.