If you read my previous post, you don't need thick gears in the first place because the carrier break for the 12 bolt is at 3.73 (3.73 and numerically higher are all the same carrier). If you have 3.73's and you are not running thick gears (which would be the case if they are the factory gears), then you should be able to use regular 4.88's. This talk about thick gears and spacers isn't really applicable in your situation.
I was in a very similar situation. I had a 12-bolt with 3.73's from the factory and decided to go 4.88's (though of course I didn't use them and they are still for sale with the master install kit). You don't need a spacer and you don't need thick gears. If you don't believe me, you can always call randy's ring and pinion.
