I've had two 9 inch'ers in GN's with stroker engines. Whatever drag there is would be slight. If I built another stroker like your planning, I would go Ford 9 inch. The 8.5 is a great rear and probably would do just fine if set up properly. I've gone that route also. To set it up right is not cheap. It would how ever be cheaper than a 9 inch. Currie 9 inch with good stuff is about $2500. 8.5 with tube braces (Most GN's that have been raced have bent axle tubes, we just don't realize it), Brute posi, Moser axles, c-clip eliminators, T/A cover, bigger wheel cyclinders, gears, labor etc. $1750 plus you furnished the rear to start. 9 inch has bigger brakes and will end up stronger. If you got it spend the extra money. May I also recommend 3.73 gears and a Detroit "SOFT LOCKER," not the regular locker.
Gary