in addition to the comment about the collection of deposits in the coolant system, if that is a stock smogger carb it, like the dist is likely from a 307, thus small and much too lean for a 455, especially if ur punchin on it, in a marine situation ur runnin the mtr in a much cooler enviroment and this may have not been a prob, also olds mtrs don't like a lotta lead timin, wil;l run hot on same even if it isn't crankin hard....
Olds, Buick, Pontiac 455's have hgh cyl pressures even at low mechanical compression, also a 307 rad is not large enough....