The GNR, like the BCDR and CBSCR, seems to have had a liking for non-standard* lengths for six-wheelers, though 27ft wasn't among them. All but the third vehicle shown appear to be of types which were either 31ft or 32ft - though the GNR also had a few of 34 & 35ft length.
I would suggest that the 3rd vehicle here is 30 or 31ft; they didn't have anythging shorter, though several pre-1876 constituent companies did - but these aren't of this type. The ones either side are maybe 32 or 34ft. The "Doncaster" design you have above is a 29ft body - nothing on the GNR(I) was this dimension, though in 00 scale it would barely be discernible.
(* Standard length for six-wheel coaches in most cases was 30ft - virtually universal on GSWR, MGWR, BNCR (MR / LMS NCC) & WLWR; not universal but common on CBSCR & DSER)/.