Just for reference, both the Dean Goods and the Collett 2251 used 5' 2" 16-spoke driving wheels on 7'3" - 8'3" spacings; this diameter equates to 10.36mm at 2mm/ft, which is closer to the 10.5mm association drivers.
The only real difference (that would be noticeable in 2mm) would be the front and rear 'over-hangs' which were 4'9" & 4'0 for the Dean and 6'6" & 4'9" for the 2251. If the chassis was sized for the 2251, it would be a matter of fettling about 3.5mm off the front and 1.5mm from the rear...
I don't have any dimensions for the positioning of the front guard irons, but looking at the pictures and diagrams in Russell's Pictorial Record of Great Western Engines
, I would consider that, relative to the leading drivers, the locations of the guard irons are not too far dissimilar for both locos. (OK, the purists may wince at this, but it could be a pragmatic compromise...).