Don't be fooled by external parts. My truck came to me with a Merc oil pan. From the paint I guessed it could be a Merc, but subsequent internal investigation proved it was a Car EAB that somewhere along the way got a Merc pan. I think that was common thing to do to eliminate the leaky clean-out plate on the truck pans.
I don't believe ECG carbs were ever stock on trucks, I believe that is a later Car carb, but has greater flow so a real upgrade. It is the same basic carb, just "grown" in the throttle plates and venturis for Y-block engines' greater HP. It should have a Loadamatic port and a "spark valve" on the throttle section (looks like a power valve, but isn't).