what needs to be changed in a 72 340 to be like a 70 340?

Few '70's with "X" heads, "X" were for '68/'69 mostly, then some "O"...The "J" heads (915 casting) came out in late '69, for '70 production year, was designed mostly for the T/A application then but eventually (with a small shift "pushrod") ended up being the most produced casting "915" in the LA series. Those heads were used in 360's and 318's till...
at least '79