I wonder if the cylinder is cracked, looks like rust in the cylinder. The bridge on the intake doesn't go all the way to the bottom on the stock 88 cylinder. Maybe you should send it off and have it re-sleeved. or keep an eye out for a 86, 87, or 88. newer style cylinders are more plentiful, have you thought about going that route? either the head gasket was toast or the cylinder has a crack that would cause a pressure buildup in the radiator, check the radiator cap as well, it could be bad.