Jeff,
One other idea you may want to look at with your SN is enabling 6th gear in the transmission. To get the most benefit from this modification you could lower your rear diff ratio for off the line starts and pulling hills. But with 6th gear enabled you can still keep your top end speed. I spoke with my local Allison service center (Pacific Power Group) about doing this with my 6400, and they said yes it could be done. They had to check with the Allison factory to confirm as some vehicle OEM's restrict doing so. Apparently International did not put any such restrictions on my chassis so Allison had no issues with enabling 6th. I also have contact information for an Allison technician that can supposedly open up 6th gear remotely if you connect your vehicle to a computer. Or you can send him your TCM.
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