First off, welcome to the forum.
The hardest part about doing the spider mod is getting the diffs out (now that it's together). I have done the mod and it's pretty much a snap. You'll need a pack of Kyosho diff pins for the MP-7.5 buggy, and a set of Kyosho diff bevel gears for the same, or a replacement set of bevel gears for the Savge. The diff housings are the same, and the pin and gear dimensions are also. The pins have the center already modified so that they'll fit together in a cross (so that they're in the same plane). Simply put the bevel gears on the cross and drop it in.
One other thing you might want to do is put silicone diff lock oil in the diffs. I run 100K in the rear and 30K in the front diffs. It doesn't lock the diffs, it makes them limited slip and better for going over rough terrain than the (pretty much) free wheelin setup that is stock.