Good old 250 GM engine. Has it been run in salt or fresh water? It's not really recommended to install fresh water cooling on an old salt water engine. (100 hours +) But I installed it on my 4.3 V6 when it was 15 years old and had 250 hours on it, no problem. But I guess that was the limit, cause I helped a friend some years ago, installing fresh water cooling on a 1989 OMC, saltwater engine. We shouldn't have done that, the heat exchanger clogged up, even though we had the heads off and cleaned the cooling channels with a vacuum cleaner.
What's gone haywire in your water pickup?