Big block cooling question
some of the older aluminum water pump housings had a problem with the interior holes being too small.
Which was disproved a long time ago as being 'the' source for overheating-issues.
But people rather like to remember myths and forget facts. Not saying the holes were not smaller than stock, but if the holes were actually too small the overheating would show up first with highway driving, not during idling.
Their waterpumps were the problem. The scrollwheel impeller is/was not peddling the same amount of coolant as the stock stamped impellers did.
I've tested it myself with a car that always was on the verge of overheating.
Tried changing housings, didn't help.
Tried a few different waterpumps and found the stock stamped steel impeller worked best and flowed the most coolant.
Maybe the factory did know what they were doing after all? :)
As for the topic-starters problem;
Are you running vacuum advance connected to intake manifold or ported carb?
