If you think about it,all of those years of experience mean very little. The metals used today are different from the ones used before. It may be the same metal but its possessing could be very different. Unless you know exactly what metal,processing and fitting he used you will better off heeding his advice.
I got my first model engine in about 1939
I have overhauled motorcycle,automobile and aircraft engines. Along the way I picked up an engineering degree.
An engineering education teaches one overall truth:You cannot be the absolute expert on anything.
I follow the makers instructions because he may have some process or technique that I have no knowledge of.