I'm surprised you haven't consulted Part Ten of the RCTS Locos of GWR series, as it contains a very clear photograph of No. 64 in its final days, with the K boiler and shorter chimney from one of the more modern side tanks.  

To clarify the story, Nos 62 and 63 were converted to 0-6-2 in 1908, and a boiler to the K Class design fitted.  This boiler was 3 inches longer than the original, and an inch or so larger, and the firebox was 2 inches longer as well. As the boiler and firebox combination was longer than the original, I am not sure why you needed to be truncated the frames.

No 64, which is the loco that appears in both the WRR photo and in RCTS, was given a revised boiler design in 1906, which was the same dimensions as the original, before its conversion to an 0-6-2 in 1911, so it appears to have retained the original smokebox design and pitch, at 6' 6" after the initial rebuild.  In this form it appears in the WRR photo. 

However, sometime after 1917 it received a repaired, larger, K class boiler and became "almost standard with Nos. 62 and 63", although it was fitted with a short chimney, which it carried to scrapping in 1923.  In this form No. 64 is seen in the RCTS photo, and it is clear that the larger boiler has been raised, presumably to clear the larger wheels/splashers, as the smokebox door is now fully circular, unlike the original, and the tank handrails are much higher than those on the cab side, which, presumably, were not altered, and there is a wider gap between the springs and the underside of the saddle tank. This is not withstanding the RCTS book quoting the GWR Diagram J pitch as remaining at 6' 6".

Without pictures it is impossible to say whether Nos. 62 and 63 actually conformed to this, but with the increased boiler diameter and other factors, it seems likely. 

As for the inside motion, I wonder if there is anything to be seen, taking into account the larger splashers fitted, and the amount of equipment on the footplate between them.

I hadn't found the small RCTS photos helpful in the past, so hadn't looked... Thanks for all the helpful info. The frames aren't truncated in length, it's the front axle box area that I cut back. Hopefully I can have a good go at the boiler changes with the dimensions. 

[later] Isn't that a good reproduction by the standards of the RCTS volumes. Scan and blow up I think. See what can be done with it. 

The motion I was expecting to see would be under the frames rather than round the splashers. The change in the reversing rod suggests the cylinders and motion may have been moved upwards, which would explain a change in boiler pitch. Altogether it seems a much more comprehensive rebuilding than I had assumed... 

Here's the GWR weight diagram as reproduced in Russell. It demonstrates the issues with weight diagrams, especially grouping era ones, in spades. I agree with Nick, the boiler pitch must have been raised - at least on 64 - so that's one inconsistency. The curved footplate over the leading wheels is wrong, as discussed, and the shape of the cab step is wrong too FWIW. The safety valve cover doesn't look like a GWR one, but the original RR ones were years gone by this stage, and it must be extremely doubtful whether the L1s, which never received their GWR numbers, were given GWR safety valves. The Ls didn't receive them until well after the L1s were  cut up. I don't think there's anything about this weight diagram I would trust without independant confirmation! I think it must have been a very quick tracing of a 2-4-2T diagram. 

This picture may help

https://ba.e-pics.ethz.ch/catalog/ETHBIB.Bildarchiv/r/101334/viewmode=previewview/qsr=rhymney

Sadly, it's of No 64, again.

That's great:-) You can even see some of the motion, which is as prominent as I feared!

Why the Westinghouse?

23 minutes ago, Miss Prism said:

Why the Westinghouse?

The RR passenger locomotives all had air brakes, but most of them had the pump on the R/H side of the smokebox. Oddly the Ls were l/h drive, and the freight classes r/h drive not sure about the other passenger classes.

8 hours ago, Nick Holliday said:

, sometime after 1917 it received a repaired, larger, K class boiler and became "almost standard with Nos. 62 and 63", although it was fitted with a short chimney, which it carried to scrapping in 1923.  In this form No. 64 is seen in the RCTS photo, and it is clear that the larger boiler has been raised, presumably to clear the larger wheels/splashers, as the smokebox door is now fully circular,

This is intriguing me more... I've tried manipulating the late photos of 64, and as well as a much higher pitch I am getting a significantly larger smokebox diameter - its a very imprecise exercise, but I get around 4'6". According to the dimensions I found in RCTS and WRR1 (at least if I'm not having a senior moment) the K boiler, although longer than the original L boilers, was still only 4ft diameter. So I wondered if the note claiming it to be a repaired K boiler was wrong, since there seems to be no reason for a K boiler to be pitched higher. That got me wondering about A or P class original boilers, of which there must have been some knocking about after A1/P1 upgrades. However they're really big at 4'9 diameter, and although it definitely looks bigger than a standard L/K boiler, I can't convince myself it can possibly be as large as the later ones. The smokebox looks more the size of a Std 3/Std 10 to me, which is about 4'5. Strange!  So *if* my guesstimate is correct it appears to be about the size of the earliest and very unsatisfactory M class boilers, which had belpaire fireboxes, so even cut down in length would hardly be a great choice under a saddle tank. What's more there would be a considerable reduction in water capacity. It doesn't feel like a viable hypothesis.
Getting back to the drawing I think I'm going to have to take an educated guess at the K class boiler in the lower pitched configuration. I wonder if the extra inches went forward, or back into the cab?

Three photos showing the life of RR 64 taken from roughly the same angle.

Original design:

Converted to 0-6-2T with original boiler:

New boiler raised up by approximately 8" based upon the change in handrail position.  I think the increased length of an A class boiler will have been incorporated at both ends (possible because of the sloping grate of the A firebox).  A K class boiler couldn't have gone backwards because the firebox grate was flat and the rear driven axle is in the way.

Only 64 recieved the bigger boiler; I have a 1922 photo of 65 with a low pitched boiler