I don't know much about serial numbers, but there are a few here who have a list of numbers. 1st, 1968 model cars had a B18 in them, so you have a B20 which somone fitted. Is it a single or twin carb car? The needles in the carbs might tell you what it had, because B18D were sometimes fitted.(less compression and an A or very rare B cam). I would see what you actually have, measure the head and gasket thickness, measure the cam lobe lift, follow the instructions on testing the cam timing and do a compression test on it. I like the middle era E blocks, a late E head for a stock stroke engine, or a late A head for a stroker.

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69 142S Overdrive + 69 164S Manual