'77 6.9, 74 240D, 96 Ram 5.9L Cummins
Many have done this conversion, it's entirely doable. You have to balance the crankshaft and the flywheel together to make it proper but some have just bolted it on. Some have had vibrations in the low 2000 rpm range up to 3000rpm as a result of harmonic problems. The driveshaft has to be used from the 240D since it is longer but either way the shaft has to be cut to length and balanced. The shifter rods will also have to be cut 4" or so to accomodate the same distance back due to the extra cylinder.