there is a very good site,
www.camaros.org,that goes into detail about the crossram and the jl8 option on the camaros. also there was a thread in here a little while back that discussed these issues in depth. I got a lot of info from "Jeff H", a member here,and others. the jl8 option will have a specfic stamp on the axle, i believe it will start with the letter "Q" with another letter designating the ratio of gearing. I think the 68 z was not a factory option, but the 69 was. you could get the jl8 setup over the counter as a "heavy duty" upgrade that would have a straight axle shank where as the factory axle would be tapered at the end. there is a link in the camaros.org site that will show pictures of the axles and explain what it all means. the crossram setup was not installed at the factory but had to be made availible to the general public over the counter due to transam racing regulations. i have seen original crossrams with the carbs. going for 15k.