I'm not sure what gpm or lsd is but I'm pretty new - one thing that will really help the experts around here answer your questions are photos! Maybe a little more details on which items you're trying to work with too. Then hopefully someone more knowledgeable than I can help you out!
While I can't answer the OP's question with certainty... @Jiggs H GPM is an aftermarket manufacturer and LSD is the acronym for Limited Slip Diff, which has various perforated shims under the sun and planetary gears in the diff to increase diff fluid viscosity by mechanical means. The end effect of this is that you can achieve higher diff engagement with lower viscosity fluids.
So I've been going over the exploded diagrams of the 8S EXB and (I'm finding this hard to believe) it appears the 8S EXB uses the same GP4 diffs you find in the 6S EXB??? Seriously? If that's the case, then just make sure to get a 29mm diff case and you should be good to go @JBOB. I'd verify that for yourself, just to be sure, but that's what I see on the exploded diagrams, much to my surprise.