Your setup is similar, perhaps some of the research I've done will help...
At 42" I'm guessing you're running 11r22.5's? I'm building a shortie right now using an M915a1 with the Big Cam III, (NTC 400), and an Allison HT-750 (direct drive 5 speed). Both the Big Cam II and III, (NTC350 and NTC400) show peak torque at 1300 rpm.
My research led me to talk to Bruce at Pittsburgh Power. They do a lot of performance work on these, and later engines. He has also written several articles advocating fast gears and direct drive transmissions instead of slower gears and overdrive transmissions. He told me (best I can recall), that if I was using a computer controlled engine I could get away with running a little bit over peak torque at cruising speed, but that with a mechanical engine like the NTC's he would recommend gearing for at least 200 rpm over peak torque at highway speed.
(Interestingly, the Road speed calculator also has text recommending 1450-1550 rpm)
When I used the Road Speed Calculator for 1500 rpm with direct drive and 11r22.5's I got the chart below. (58.7 mph at 1500 rpm)
I decided to swap in an air suspension and 3.08 gears to replace the walking beam and 3.88's
Speed Calculator:
Big Rig Road Speed Calculator: Semi Truck Road Speed Calculator
Articles:
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