the big thing going from a mechanical engine to an electronic trans will be throttle.. I think both the DD and the ally 4000 are SAE1, so you should be good there as far as mechanical mating.. you will need to have a TCM made and programmed for your particular RPM and speed applications.. the siomplest way is to find a standard TCM that has its 100% throttle upshifts at just below the redline of your engine.. and a shift curve that is best for a V8.. 8V92's are common so im guessing an allison dealer like stewart and stevenson who does custom work and find a calibration CIN that would be nearly perfect out of the box..
throttle input... you'll need a box that will take a mechanical cable throttle and turns it into J1939 that will talk to the TCM.. the modern 4th and 5th gen allison TCMs can do OK with just throttle input.. when you enable the fast-learn routines riught after the swap, the trans will go into learning mode and learn not to "flare: or "bind" shifts..
there are quite a fewthrottle adapters on the market. here is an example of one.
CAC THROTTLE POSITION SENSOR | CAConversions | Custom Automatic
the throttle adapters for the 4000 series will be the same as the allison 1000 swaps sionce it simply takes TPS and turns it into J1939.
I dont know much about the DD, but if you are new enough to be a DDEV IV OR DDEC V you may already have a J1939 interface available which could be connected to the CAN 1 port of your allkson TCM and send throttle from the engine.
as far as which exact allison model, it gets tough.. you may have to look at other similar applications and see what was used. allison does a lot of OEM spec work so there are a lot of SKU;s. not knowing what the truck is you are working on it becomes harder to recommend the right trans. thats where you'll see a lot of difference in the specs.. ie the 4000 base series is the generally available unit.. but the variants like 4430,4710, etc are specialized units.. you also will need to do the math to get a Torque converter to suit the stall you need. that DD likes to rev a bit i'd go with a bit higher stall converter, esp if you are running heavy..
are you planning to source used parts or looking to buy new?