340 x heads

So yesterday I found out that my 360 has a set of x heads on it. I don't know the date of casting on them so don't know who to sell to if I sell them. But I haven't had any luck finding anything about them. As far as I can tell, they are pretty much exactly like the J, O, U heads but without hardened seals and 2.02 valves instead of 1.88s. So what do I need to know about these heads? rocker ratios, amount of lift possible without machining the seats, porting tips, any weird thing about them that is different than other LA SBM heads? Answers would be greatly appreciated.