Swap in the stock 48RE 4spd auto trans. There have been quite a few people changing transmissions on the 3rd Gen Cummins. Some of them are pairs of guys getting together and swapping each other the necessary parts.
You could even get away without swapping the steering column and doing a floor shifter.
What your husband is thinking of doing has already been done numerous times.
The Allison trans WILL require a standalone controller, and that will depend on what model you end up with. Using the stock 48RE will be a direct bolt on because the truck is already designed to accept that trans. Any other will require more work for crossmember, adapter(s). If truck is 4wd, using a different trans complicates and increases the cost of the install.
