If I was you and you would not want to continue to fork out mid range prices - which added up are more than the heavy duty parts; I would get the heavy duty final gear for the engine you have lol. From my understanding its more powerful than the 18ss, so I would definitly get them. You should NEVER over shim your gears as the added friction and lack of play will kill them. In saying that, you can create a different atmosphere for the diff in terms of lubricants to help ease the pressure. You can use various weights engine oil or motor bike oils with different viscosities. I have set mine up with a car diff oil and my MT2 diffs are 1/4th filled with this oil so that they are always lubricated. My shims are standard shims with 1 extra...thats it. Bearing in miond that these will not last forever but if you train your self to ease off the throttle so its NOT "WOT" all thime when turning corners, your diffs should last longer.

On pages roughly 200 and below there abouts there was a poster who did the same thing as I did by putting and experimenting with various lubricants and viscosities with his diffs, and he even put a quick refil hole in the top - which is very good if yu have the alum ones and you can fabricate a tapping for the cap; not too hard.