Why twin 6-inch guns were possible but awkward on early Town cruisers

Could the early First World War Town-class cruisers have been fitted with twin 6-inch guns?

Yes, but only with considerable rework; twin 4.5-inch or 4.7-inch mounts would be substantially easier to integrate.

The initial question is ambiguous because later Town-class cruisers could in principle receive later twin 6-inch automatic mounts. The follow-up clarifies that the subject is the earlier First World War Town group. Twin 6-inch guns could be fitted to those hulls, but the conversion would require substantial structural and arrangement changes. Twin 4.5-inch or 4.7-inch mounts would impose a much easier modernization problem. Feasibility therefore does not make the heavier option the most practical refit.