Ok, well...GMC casting number of 6107326 means it is a 228 (which we knew) but there should be another number (looks somewhat like a plate with two screws on each end). This number will usually begin with a letter (could be anything from A to L and this tells the month) followed by a number (will be 1 to 31 and this tells the day) followed by another number which stands for the last number of the year. I think this is on the same side of the engine as the distributor, but I won't promise that...it has been decades since I went through this hunt. I will tell you there is a problem with the final digit. If if is 0, 1, 2 or 9 then we have to guess based on the number near the distributor (and I can do this...I have saved some from GMC trucks I've owned or worked on).

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end