360

No 340 ever came from the factory with closed chambered heads and yours aren't closed chambers either. They are virtually the same head the 340 used except with smaller intake valves. The compression ratio is in the neighborhood of 8 to 1, 8.5 to 1 max. 360's were first used in trucks in the early yrs then they started using them in cars in 72 or 73. They were not considered a high performance engine until about 74 when they started putting a 4 barrel carb on them. Even though they had smaller valves and low compression I've seen some of the early 4 barrel models run fairly strong.