750-850hp......IMO you are flirting with disaster should you choose a 31 spline, those numbers justify a 33-35 spline. Sleep at night and take Strange's experienced opinion.
I am sure when you called/talked with Strange, the sales/tech asked all the necessary questions prior to his 35 spline recommendation? If so, why would you think Strange (axle GURU's) would steer you wrong? Strange is a solid source and if they are recommending 35 spline for your specific application I would not look to other less knowledgable sources.
Many on this site have individual opinions (as myself) but very few have more real world experience than Strange......listen to the professionals