That would be a pre-70 oil cap your looking for.. Never seen a 70 BBB with one of those, have seen them on 67 430 engines... Pretty sure they went to the later style pictured above, for the 1968 model year, on the 400/430 engines. Both styles are available in reproduction I believe. Either style, used, is most likely not going to seal, they had a fiber gasket on them, that hardens over time. See it all the time on the dyno.. little drip from the oil cap. JW
