Does this sound about right ?
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Thats on a modern engine with modern coolants, you have neither.
This is a break-in you don’t need nor want to run the engine so hot. Under a 15 pound cap you can run coolant to 253 at that pressure that is the top limit. Trouble is if there is a leak of pressure at that temp or actually any over 212F there will be bulk boiling to an explosion of coolant, this will stop your break-in in its tracks. The whole point is to get it running and keep in running not to fix accidents of cooling, fuel, oil or tuning problems so large they force a shut down.
Actually since you’re outside in winter temps the chances of loosing control of coolant temperature is much reduced If not eliminated.
As I’m sure you found out by now in this project that you're up against Murphy’s Law where “Anything that can go wrong will go wrong, usually at the most critical time”. Your whole job is to prevent this description of nature from taking its course.
50/50 is the typical mix of coolant and distilled or deionized water at 15 pounds per inch squared the boiling temperature of this brew is 253 at atmospheric pressure; it’s 225 against 212 for pure water. At 253 under 15 psi of cap pressure the loss of that pressure at that temp results in the immediate change of state from liquid to a gas resulting in an explosion that will take your skin off if your in the way, and where are you at this time but head first above the engine, yikes!
Additionally we’re talking about bulk coolant temps which are crazy enough, but wait, there are other surprises inside the cooling system called “nucleate boiling” where immediate local temperatures far exceed the bulk coolant temp. These are mostly the head’s exhaust seats especially where the exhaust is immediately adjacent as in the middle of the SBC head and the spark plug bosses which if you remember looking at the gasket side of your heads each and every spark plug boss has its own coolant passage. When the bulk coolant temps get up around 220 you can rest assured that these nucleate boiling points are boiling into the bulk coolant. This allows local overheating causing these areas to expend against cooler areas of the structure around them. When this strain exceeds the material strength you get a crack something pretty common on GM castings in particular but nobody is immune.
So the morale on this story is to go in prepared to keep coolant temps well away from extremes yet hot enough to get to running clearances. The sweet range is about 180 to 200 with 50/50 coolant under a 15 or so PSI cap.