Have you checked the flywheel bolts for the proper torque? Your symptoms sound like a classic main bearing knock. I drove a 1960 Rambler American flathead 6 from Central California to Nashville, TN. with a similar knock, and discovered the main bearings had about .002" too much clearance, after an in-frame rebuild before the trip started. That one also wore out a new set of piston rings in 2500 miles due to inadequate cleanup of the cylinder walls after a honing job. The last couple of fillups on that trip, I had to stop midway and add oil. It was using 3 quarts of oil to a tank of gas!
Jerry
