BTW, many here on BITOG have never seen a disembeled engine that has ran for 50,000 miles or the equivalent hours, or more with leaded gas. The lead can build up around the valves and when the build up gets thick enough it can prevent the valve from working properly. About the only place to see this now days would be when an airplane engine is taken apart.
Also, probably because of the effects lead had on the valves and other internal parts, and the poor regulation of air to fuel ration that a carburetor with a float has when subjected to the forces exerted on it while in a moving vehicle, engines of the 60's were only expected to last 100 K miles, and even with 3K OCI, you were lucky it you managed to get that many miles out of them.