One of the great things about Pontiacs is that all the parts from different years basically interchange. But it isn't perfect either and doesn't always work. Different years had different "this and that" bolt holes and add ons etc for different options. Some might be there on yours but not used. Curious what number heads do you have? They should be #17 on 1968 350 2bbl. (best 350 head by the way!) Possibly even #47 in a late run. But often they were swapped out when they got tired. Could be anything?