I would stay with the timing issue. When the gas comes out of the carb, that means that one of your pistons is shooting air through an intake valve that should be closed. Have you ever replaced the timing chain on this engine or how many miles since rebuild? You also say the timing has been checked but how did you do this? The harmonic balancers are known to crack and move, so the mark might not be where it should.
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69 351W Sportsroof deluxe
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