The H head was very rare. It came out when the government wanted more horsepower and GMC built the H head in 51 and 52 while they were developing the 302. I have in my notes that the H head was casting number was 2193417. But I am not real sure about that. I think the H head still used a domed piston. I know it had the large intake ports that used a single barrel carburetor. A 302 manifold would fit but it used a 2 barrel carburetor. In civilian use it used a 3 bolt Holley and government applications used a 4 bolt that accepts a WW Stromberg. The correct manifold for an H head is a single barrel carburetor. If I remember correctly GMC did not change pistons for the H head. The H head raised the compression for the old dome pistons and used larger intake and intake ports. 2136477 and 6107412 castings were for the domed 228, 2194819 were for the flat piston 228, 248 and 270, 2193417 were for the H Head and is for all 302 engines. All of the GMC heads use the same bolt pattern, but the ones for domed pistons have to have domed pistons or you will have hardly any compression. The ones for flat pistons need flat pistons because the domes will hit them. See what the casting numbers are on the heads you have. I only had one H head in my lifetime and it stayed on the engine until the vehicle was stolen. So I really don't have first hand knowledge if the 270 with the H head had a flat or domed piston. But i remember correctly everyone was looking for H heads 60 to 70 years ago to get more power out of their 228 and 248 engines . Post some photos of the casting numbers and the shape of the combustion chambers. The combustion chambers are drastically different for domed pistons and flat pistons.