[pippintook]

The timing cover tab on my 78 W72 has the numbers 4-0-4-8-12. So, 12 is the last number, but, there are hash marks for 14, 16, 18 and 20. I would think the timing cover for a 77 W72 would be the same. So, you can set the initial timing to 18 if that is what works for the engine.
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I would not be too concerned worried about what an original W72'S initial timing was. I would try and find the best initial timing that works for your combination. Your combination will be affected by many variables: amount of mechanical advance, rate of mechanical advance, amount and rate of vacuum advance, ported vs non ported vacuum advance, compression ratio-head porting, camshaft, exhaust, carb, carb jetting and mods. etc.
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I would experiment with initial timing and see what works best for your combination of parts.