Check Your Ride Height
I concur with checking your spring center bolt. The head on those bolts are usually to short to accept any degree shims along with the perch mount on your rear end. Not sure how you ended up with an 8 degree requirement, but before you make the leap, check your ride height (level the car) and see what your rear end is actually reading from the pinion. As far as stacking a 3 degree and a 5 degree, I would be very cautious and personally would not do that. If you don't have the time to the job right the first time, where will you find the time to do it over again. Rework is way to costly.