I second the guides/stems as suspect. You may also want to take a peek under the valve covers and make sure that the oil drainback holes in the heads are not clogged up with 41 years worth of gunk. Since the rockers and rocker shafts receive their oiling directly off the main bearings it's easy to get an oil flooding condition up top with even slightly worn rocker shafts. This can cause oil burning even with brand new rubber umbrella seals