overheat 454 rebuilds help

Ok need tips on clearing a air block in the fresh water side of new rebuilt 454 gen4s have great raw water side, about 5gal flow in45-50 sec at 800 rpm. along with rebuild on longblocks i have also replaced with new the trans/oil coolers. freshwater pumps, oil lines from remote to cooler and block and all risers & elbows on both eng. manifolds looked pretty good so kept them. The heat exchangers both are clear and wide open on raw water side of tubes and new thermostats last summer. i opened the vent on thermo housing when filling for first time with 50/50 antifreeze as directed by service manual. At the start of all this the return hose from HE back to water pump was cold to touch but after running and opening vent again now it warm to touch but temp gauge still shows over heating? on both eng timing is good also new temp senders. Any tips or other thoughts ?
Also how high a temp is to high even for a few min to get to before shutting them down and waiting for them to cool before next try ? THANKS ALL

Hard to say with certainty. I don't recall having to spend much if any, time bleeding my genIV. I will say that the exh manifolds heat up pretty good before the thermo opens. Steam and boiling water is too hot. Before that, at warm up, you may get some short spike at the thermo, but not usually more than a couple of minutes, and then the coolant should start moving.

im trying not to let them get above 180-185 for more than a few sec as im not relly sure whats a safe upper temp limit. thanks for your thoughts

I have always had to bleed out the freshwater side by running thru a few heat up - cool down cycles, mostly driven by how much air you can get out on the initial fill. An IR temp gun is best for accurate temp assessment. The other item that would help is to ensure the HX opening (filler cap) is the highest point in the system....the mounting bolts are in slots and seem to remember they are a few inches in length....