If the inside lining is deformed then it has probably been cooked before. When you replace the impeler check for all the blades that are missing, they usually end up by the thermostat and if not removed they will continue to cause you overheating issues. Although it doesn't sound like you over heated it for very long, it may have started the first day so be cautious the next time out. 20 yards from the shore should be no issue with the heat, if it ran until it over heated to the point it would start the day before then there may be some internal damage.
More GT40 info on PN, we like carbs and wood on CCF, much easier to do mods that way.