I have a freshly rebuild 1968 460 block that was machined with new pistons, cam and almost everything accept the crank. The break in was great and it was idling after its break in oil change. It was sounding great, then I backed up ten feet and it stalled. Then I pulled forward 10feet an their was a metal on metal noise and the engine would not turn over. Any ideas on what could cause it or how to fix it? Any help would be much appreciated.