The 1:1 final ratio Allison's were never geared for that speed. They were geared for about 60 on the governor. The speed of 70 to 75 is going to kill your fuel milage. You could probably set up the drivetrain to cruise at 65 with some economy but you are going to have to start out in low a lot and manually shift the trans. There are a lot of factors that go into setting up a truck and if you get one wrong you end up with a fuel guzzling dog.
You have limited horsepower. The engine needs to live in its sweet spot for economy. Usually you have to decide on either economy or speed. That is the reason that the fleet trucks are governed around 60. As speed increases drag increases exponentially.
You would be better off with a medium duty.
Here are some trucks that could be duplicated to better fit your needs with a lot less work and money.
I can not pass up the mack but it is too big for you needs.
Your wife might like this
If you really want to persue this idea you would need one of the Allison 6 or 7 speed transmissions, but they are electronic and really expensive. The best option would be to teach here how to drive a stick, and float gears, an use a 13 or 18 speed overdrive,