most magnum engines will run just the same milage as the older 318 and 360's
the 4.7's are hit and miss, some are really good and run for several hundred thousand miles, others are dead before 100K
same with hemi, hit and miss; truck engines are horrible on fuel, no better then 318/360 worse in most cases (if you get 20 its downhill with tail wind)
3.9 works alright but its not a powerhouse.
if I had to choose any and had enough I'd buy a truck with a cummins 12V any year from 1989+