My feeling is that the system in the XJ's are already inadequate enough, hence your possible decision on relocating it! I would go with more surface for faster heat release (transfer). I once saw a Land Cruiser with an additional 18"x18" roof mounted radiator. I do not know if he had an electric in line booster pump or not? I know some race cars with rear mounted radiators have electric pumps for 2 reasons H.P. and flow! You should consider putting an in line pump for the added head pressure the extra capacity of water and distance will create!