My old Ramcharger had the double cardan in the rear as well. I had about 7" lift. I had the pinion about 3 degrees down from the pinion to transfer case line. The pinion ended up being 10 degrees up. As you mentioned, the fill was below the pinion bearing. The truck was off road only, but during long hill climbs, slower trail speeds, high torque with the 440 in low range, I didn't have pinion bearing problems. With road speeds, I would think there would be enough gear oil flying around to keep the pinion bearing happy.
If you still have concerns, you could get a fab cover and have the fill plug wherever you want it. Just a suggestion.