The warranty on the drivetrain was 5 years or 50,000 miles. So if I replaced Your 68 engine in late 69 or 70, as long as it was not 50,000 miles, or the year 73, it could have a 512 block casting. Do You have the POP or invoice? The engine suffix should be on it. It should be a 4 bolt main with solid lifters. NICK L72