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All Olds heads from 1965 on will phyically fit each other engine but with some differences, such as a 403 has 1/2" bolt holes as do the diesel but all other's have 7/16" bolts. Chamber and ports size differ between big & small block engines but the heads will bolt on each other, doesn't make it right though. Now getting back to the 425/455 heads, they are both big block heads, thats good. All 425 heads will work on 455's. The odd-ball of the big blocks is the 1965 and early 1966 425 engines,(non-Toronado) these engines were 45 degree'd cam shaft'd. All after were 39 degree'd, so if u put the mid-1966 and later heads on a pre-early-1966 425, they say u have to elong the pushrod holes in the head a tad so the pushrods don't rub. In my own experience I have found this untrue but the "heavy hitters" tell me I am wrong. So, anyway the heads u have on your 455 will be fine. I would almost bet you have a 1973-76 455 engine. These came with "J" heads, smog head, the worst flowing head. Everyone gets a set of early heads to boost HP via better flowing heads. The Big block Heads have a Capital letter in front on #1 sparkplug, "A" heads are 1965, good flowing but have a one year only valvetrain, "B" heads 1966, good heads, "C" or "Ca" 1967,1968,1969, good heads, "E" 1970, good heads, "G" or"Ga", 1971,1972, good heads, 1973 and beyond are "J"'s. There are also "D", "F","K","Ka", These are heads made in small amounts. Later, Ken
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