I wouldn't refer to a 1968 manual for a 1961 engine. the 235 and 261 ended in 1962 with left over engines used in 4x4 in 1963,the end of this generation.If you purchase the 1961 shop manual,it will show you everything needed to rebuild engine.Reference the 1961 shop manual,it will be your friend.

1951 3100 Chevrolet
1951 Chevrolet Suburban Carryall
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