The Excelsior is the worst in distance.
Unless, of course, the main reactor is in the pod between the nacelle pylons.
The Excelsior and Constitution limit power conduit exposure to Engineering hull and neck. Reliant runs into the single hull.
And Reliant's hull configuration means the conduits do not need to run directly through the engine room in order to reach the nacelles. Arguably, they do not even run through an inhabited section of the ship since the shuttle bays and their machinery are located in that area; radiation leaks in that case would only affect the relatively empty aft compartment and would be that much easier to contain (meanwhile, most of the engineering spaces are FORWARD of both the reactor and the conduits and can likewise be isolated from them in the event of an incident).
So without having to babysit a potentially troublesome power transfer fork point, Reliant can keep its engine room just aft of the saucer like a normal starship
, allowing for some degree of survivable radiation containment in the event of an accident.