You have lots of options.What is more appealing to him?And which one will take the least amount of time to sort out??

A 5.9 magnum swap is a cool idea and a matching OD tranny isn't a bad idea.
With modern aluminum heads and a stroker kit or 360 shortblock will make plenty for what he wants just don't over cam it.You can still swap n an OD tranny and mileage will greatly improve.
Old school may be old but it still gets the job done with some modern additions.