Lakester,
I'd perform a leak-down test to verify the condition of the rings. You may have something as simple as an intake manifold gasket leak that is allowing engine
oil to be sucked into the intake ports under vacuum. You mentioned possibly having to sleeve the block this time around? How many times has it been bored and what is the bore over-size now? A .005 custom oversized piston is less expensive and more practical than sleeving all eight holes. A quick sonic check of the bores would verify if there is enough meat for an overbore.
hth,
--Mike