RE: Truggy Gearing
First, I'd lose the center diff and get a solid center spool. A center diff will constantly unload with MT tires unless your using concrete for lube. A torsen (center) is NOT recommended either, it will only get thrash.
I've managed to put the MP spur on nearly all my truggies, but its usually not easy due to clearance issues. See if you have 1/4" or more space all the way around your current spur, if not, you'll need to modify whatever to make it work, most buggies (originally) will need new tranny plate and spacers to lift the tranny assembly up accordingly at a minimum. Making truggies from scratch is mainly alot of planning, trial and error and is not something I'd suggest a newb to tackle unless they have good mech skills, access to some tools like metal band saw and drill press, and dont mind the challenge. A jig saw, drill motor, and dremel tool works but requires alot more work so it doesn't look like a hack job.
I think the MP spur could be installed on your spider diff for initial moc up
and testing.