Well this is critical information you have let alone confusing and not very reliable (the coding that is)I thought there should be a seriel number also. If it is a 1979 then it has 1/2 inch head bolts and the 1983 has 7/16 head bolts at least by my research. So if I did a 4.0 head conversion and have a 1983 the block may have to be drilled out to 1/2 inch. This all of course is subject to a compression check of the engine before doing th conversion. I think I should have at least 125 lbs in each cylinder.
As per this link- http://www.off-road.com/jeep/tech/engine/amc258.html

Do you know of anyway to determine which year engine it is, short of pulling a head bolt or valve cover or???