I'm just about to convert a 26 to a 27 (body is currently sitting in paint stripper) and it's definitely worth getting the etch from PRMRP. As well as the headcode box it also has new tank sides, fan, snow ploughs, side windows, steps over the buffers etc.
They also do castings for the roof vents.
The only bit I'm not looking forward to is sanding down the raised part of the roof.