Olaf:
This was one of the problems which I had with my previous engine. The builder in Maryland did exactly what you describe, all the while denying vehemently he was doing it. The engine overheated at virtually any outside air temperature above 60 degrees F while driving below about 40 mph, because I couldn't get adequate coolant through the engine. I also had additional problems with inadequate
oil pressure. The end result was a spun rod bearing, a destroyed crankshaft, a cracked block and a long legal process. IMHO, the block is likely unsuitable for street use. If you do try to salvage it, make very sure you have it checked for cracks following whatever work you do. The temperature gradient between the cooled and uncooled portions of the block may have resulted in cracks in the uncooled lower portion of the block where the cooling jacket meets the vertical exterior sides.
Good luck.