You will have to pull the headers loose to check inside of the exhaust port. You can feel the hump and a hole for air to mix with the exhaust gasses. Probably what I would do is to find a contact gauge and just find out what the head temps are if you have that concern about differential. Just a word about those '76 heads - they are close to the worst ever used on a Ford as they were trying to reduce emissions and Hp to meet the fuel crisis so they made the valves smaller and reduced the compression ratio. I don't have my book of specs close, but not their best choice. A nice pair of iron GT40P from a 5.0 Explorer/Expedition would add new life to the 'beast' and the base for my Avatar's engine heads. (I had a '76 F250/460 that was a California truck - now, ya wanna talk emissions junk that disappeared)

Cat diesels are a great power plant - if you want to watch a lump of yellow iron all day.