Keep in mind, he's not using this for city driving. On the highway, auto with TCC locked vs manual doesn't make much difference, it's the overall gearing that counts.
As far as lugging it down to 1000 rpm, that's too low to have enough power to do much with in stock form. Plus, it may actually put him below the good part of the BSFC curve. To even think of playing down there, engine mods to increase low end torque are likely needed. In my Jeep, it starts dropping off pretty hard below about 1100 rpm, and my engine is a good bit bigger than his (5.9 liters).
I'd say he should shoot for somewhere in the 1300 - 1400 rpm range, 1500 was the absolute max. After a little research and math, with his stock AODE transmission, stock 215/70R15 or equivalent (26.8" tall) tires, and a 2.73 rear end gear, he will turn 1375 rpm at 60mph. That and aeromods could quite possibly get him very close to 40mpg.
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Call me crazy, but I actually try for mpg with this Jeep:
Typical driving: Back in Rochester for school, driving is 60 - 70% city
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