Engine only has 2000k since rebuild. Mechanic replaced rear main anyway, but it's looking like the '73 pan on the '68 block may be the problem. I originally ordered a 68 pan gasket, but the '68 used holes to align the end gasket Our 73 pan has no holes) so we had to order a 73 gasket kit which used "shoulders" to hold the ends in place. We're wondering if maybe the 68 and 73 are slightly different diameter?