so you are talking about a bronze guide liner type deal-----------guide in head probably was repaired with a liner but was too oversized for the liner and/or liner was not broached into the head guide. If this was the case you can repair the head with a .502 guide-


In overheating cases(no matter what the cause) engines with two exhaust valves together (sbc, olds,etc) suffer pretty bad in those cylinders---------exhaust/intake valve placement has always been a big item in engine design---people who do the heads and people who do the intake/exhaust manifolds don't seem to get along---------check out BBC!!!!!!!!!!