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building a 9"

hey guys. im looking into building the 9" i have in my 77 f150. though i shelled out my trans the other night, turns out after going back and looking playing around with it, i lost something in my rear axle.

my question about building a 9" is should i go 31 spline or 35 spline with good yukon or moser axles. i run 285/75/16" and don't plan on going any wider or taller. i want it to able to handle 550ft lb and 450hp, dont plan on building my 429 past that. im plan on using a trutrac diff and 3:50 gears. i use this truck to do some towing in the 5,000 to 7,000lb range, and for fun driving and occasional street racing. i dont off road in this truck, but i does see some mountain roads. truck with me and a full tank come in at 4900lbs

any insight how well either one would stand up to my usage would be great. i sure dont want to have to build a rear end for long time given the prices.
i would go with the 35 spline simply because theyre stronger.. while the 31 would hold the power your going to put down the 35 are just that much better..

either brand is good for the axles..

when you say some towing how often would define that? while 3.50 is a good street gear for good mpg if your towing frequently i would go with the 3.89 gear.. its right in the middle between mpg and the performance 4.11 gear.. if you were towing heavier, daily, or wanting grand street performance id suggest the 4.11 as it takes more stress off the drivetrain. which for when you want to do that street racing the 3.89 will help you get moving a bit quicker... as far as mpg.. the mustang guys figured out a long time ago the difference between the gears came out to a 1-2 mi difference per tank going from 3.55-4.10's on the 8.8's.. so your not going to see much difference there... plus itll help you get up and down those mountain roads easier especially with the trailer behind you..
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i rarely tow, and this truck see's everything from mountain roads to long sections of freeway but mainly works are a run about getting stuff my car wont and cruzing. so the 3.50's work well for me matched with a np435. or at lest i think so and i like the fact i can get 11.5 to 13 mpg.
sounds like the 3.50's would work better for you then...