My 68 327 manual is emiting grey smoke at idle and espeacilly when I get on it. I fear I may have some bad rings which means a major repair job. I see in eg Jegs that after market engines are available but I am unsure that any of the small blocks that are sold can just be swapped in without difficuly of things not lining up eg. motor mounts, connection to the bell housing etc. Anyone have experience replacing their old motor with an update one?