Well, I am NO WHERE near where you are in HP (but give me some time
)
I have rebuilt my rear end (after I blew it up
).
You can easily tell if they are 28 or 31 spline by simply counting the number of teeth on the axle. Sometimes it's hard to find the maker of the carrier just by looking at it, if it doesn't have a brand on it.
I have FMS 3.73's and an Auburn Posi diff. I love it and it gave me lots more torque to the road then I have in the past. I can spin my tires longer and in higher speeds/gears. I know that is not a good thing but my street tires suck....
I bought my carrier from Tommy Vaughn Ford (a Ford Racing local dealer). Mine was only $388. I dont know if it would hold the kind of HP you are looking for but then again, Auburn is awesome!
Hope this helps...