May I add my six pennorth? The J1 was indeed a rebuild of the E1 tender locos, originally made by Sharp Stewart of Manchester. All my authorities state that that the driving wheels were 5' 6" and ponies 3'6 ". It was 7', 9" betrween drivers and 6'6" between drivers and ponies. The rear ponies were placed in a bolt-on addition to the mainframes. One of them, I think number 47, retained its trumpet shaped safety valve cover, while all the others got Ramsbottom safety valves.There is a lot of external rivettingon the tank and bunker sides, as there was on the Neddies. When I had the latter 3D printed, I also had some half etched cab sides done in nickel silver to simulate this but I expect there are other means to do it. Hope this is helpful,
Paul R