A 7.5 ford rear would be fine as well. I'd shoot for 2.73 gears (stock ratio in most Mustangs). That should get the rpms down. I'd think you'd want cruise rpm to be under 2k personally. 1400-1600 and the motor should be near torque peak.
Here's a guy by you with the 2.73 ford 8.8
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Lots of Mustang Parts