Clint;
Just did this on my 261 rebuild. Used GM's EOS supplement (not EON) during the rebuild and then poured the rest into the crankcase. I used straight 30wt Valvoline during the start-up after checking it's zinc properties on the web. Also put in some LucasOil as an additive. Like SW and Hotrod said, don't be scared and don't second guess yourself. If the static timing is good, valves adjusted close, all fluids good and plugs wires correct, get it and let it fly! I cranked mine about three times for 15 to 20 secs each time until the carbs were full and she started right up! Kinda of scary listening to that thing running at that RPM right out of the gate but it does work. Make sure you have some cold Coronas to celebrate when you shut her down. Good luck!
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