Does it have GMC on the valve cover? Does the side/push rod cover come up to the valve cover (around the spark plugs)? A 228 is a GMC only motor, full-pressure lubrication to the rods as you stated, has a short side cover and GMC on the valve cover. The serial number also usually starts with the engine size (228). A 224 looks exactly like a 216; valve and side covers.
Have you visited the oldgmctrucks.com website?
