You do not need 5" to run 37"s, keep as low as possible.
Their 5" is not like Trail Gears. I'm running their 5" and my 35s rub on full stuff. So if he isn't wanting to trim then he might even need to go taller. If he wants to trim then I would say the 5" or the 4" is the perfect spring.
My All-Pros are great springs. I should have gone with 6" in the rear as they sagged out but I have a runner so it is a different situation for me than you. You shouldn't have that problem.
