Do what you wish but don't let anyone kid you into thinking they are the same truck based on axle ratings. The 2500 is not a 3500 with a couple of leafs missing from the rear springs, it has a completely different rear suspension.
I asked the question that if they are so close to being the same, why doesn't the 3500 use coils as well to give a more compliant empty ride? Not getting any answers, I will offer one possibility. Because 2500's are restricted to a 10,000 lb GVWR, there is no need to provide springing that will accommodate a 12,000 lb GVWR, even though they use the same 11.5 in AAM rear diff, therefore they can take advantage of the better ride that coils offer.