Oil consumption during high-vacuum conditions like coasting at high RPM in the lower gears is almost always caused by piston ring problems. Was the block bored during the rebuild, or were the rings just replaced without a rebore and new pistons? Cylinders wear most in the top 1 inch of ring travel, so a ring is constantly changing diameter to try to stay in contact with the cylinder walls if the bore is even tapered a little bit. At 2,000 RPM, the rings are flexing 66 times per second. It doesn't take long for a ring to lose enough tension to let oil slip past it and into the combustion chamber. Reboring and fitting oversixe pistons straightens out the cylinder top to bottom, and makes the rings last a lot longer. I will not re-ring cylinders that have more than .005" taper.
Jerry
