Most all motors have thier "most likely to fail spot when overheated". (ie failed head gasket, warped heaed, cracked head, ect.) what is it on an 86 460 motor? It happened a few years ago and slowly lost coolant. (none in the oil) for a few years block seal made everything ok. It is now overheating again after 20 minutes on the highway. does not appear to have any gasses in the coolant.

 

could be any # of things not even related to your original problem !
block seal stuff was never made for a permanent fix !
hows the water pump , the radiator , the fan clutch , fan ?