No, it wasn't a rebuild, per se. It had been rebuilt at one time, but when I bought it it was in a running '55 that was being upgraded to a 350. My truck had a '52 in it when I bought it but it was blown up. It had water in 3 cylinders. I bought the '62 as an upgrade to a full oiler. I had to swap the ring gear to use the 6 volt foot starter which I wanted to keep. I also had to modify the water pump with the shortened shaft, but I never went through the bottom end. I did pull the head, have it milled to make sure it was flat and put on a new head gasket, set the valved, added a new fuel pump, swapped exhaust/intake manifold, put a Carter rather than a Rochester on it, etc. I am afraid you are correct that there is something wrong in the bottom end like bad bearings or missing/improper shims.
