650 is what i was told also about a stock RB block also. i know there is a little more webbing area on the low decks. there is a guy from sulphur louisana with a green dart i seen at the evadale track with a built 500 low deck and he also filled the block to the freeze plugs. dont know exact figures but he was makin some serious power for a street car that was natural aspirated. i asked why the low deck and he said for the strength. later i plan on gettin a after market block but i have to do with what i have for now.