The main difference between early late 2wd heads, is 3 door and sapph.
3 door heads have a closer fitting area around the stud/bolt holes, meaning they take a smaller headed bolt or stud nut/washer combo than later sapphy heads(2wd) that have more space around the hole to take a larger stud/nut or bolt, so better clamping force more evenly distributed.
4wd heads have different INTERNAL waterways, between the block and head, 2wd have long slits, 4wd have two round holes so support gasket better, for more power and so are favoured.
Port shape is SLIGHTLY different, but not hugely so, and once ported neither head has an advantage over the other save for the better support of the later 4wd head gasket.
4wd sierra head not quite as good as escy head, but so little in it, escy is slightly better on emissions as a result of burning characteristcs.
tabetha