Poke a wire through the crud the machine shop didn't get out of the bottom of the water jacket. You got panicky and shut things down too soon- - - -a couple of minutes more running, and the temp would have probably stabilized. Were you running a big box fan or a squirrel cage blower to the radiator?

Most block cleaning these days doesn't use strong enough chemicals to dissolve a few decades of rust and crud, but all that should have been double-checked before you put the engine together. Ask the guys who did most of the grunt work on John's 261 about the extremes we went to getting the water jacket cleaned out- - - -it was an epic effort!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!