Well as I understand it running without exhaust is not good. the reason I think is because of vacuum both in the intake of fuel and the exhaust of hot burnt gas. The exhaust manifold is a vacuum chamber. Anytime one cylinder fires and is exhausted it creates a vacuum on the other cylinders. this helps exhaust the hot gas. Where the warped valve comes into play is not when the engine is running but when the engine is shut down. At any given time either an intake or exhaust valve is open or both valves to some extend. Not to say it could not happen running if their is no cooling your headed for a melt down, but what happens is when you shut down the hot exhaust heats the air in the cylinder causing is to expand. the expended air flows thru the partial open intake valve causing a vacuum drawing colder air into the cylinder, cooler air warps the valve. Ok that said, running no manifold at all not very good, running with manifold good, low rpm short time 3 to 5 minutes no not temperatures, manifold with short tail pipe 2 to 4 foot Ok I guess keep eye on block temperature, manifold tail pipe muffler best, I thing back pressure comes into play here also, manifold tail pipe muffler get hot so if cold air gets sucked backed into engine it's from the hot exhaust not outside air. Yes rebuild fresh engine run hotter than use or broke in engine, Best way to break in new rebuild is with every hooked up, this way you did everything you could to insure it will run without problems, peace of mind.
When I did rebuild on a engine I run 3 to 5 minutes with exhaust manifold hooked up no hot temp just warm to see if any weird noise, then install with full hookup everything,do break in rpm 15 minutes rpm and 15 minutes higher rpm, shut down cool engine startup engine drive moderate. Well could be wrong but safe way is full hookup Jbennett
