Watching this with great interest as I am (still....) putting the finishing touches to two of these locos. (thread here )
It is certainly possible to produce a pleasing model from this kit, though there are a few things to look out for!
It is difficult to get the footplate to sit just right - there are so many different curves and levels. The cab and firebox area can be used as a reference point, and the front frames / smokebox saddle adjusted until it sits right on the chassis. Check and recheck as you add parts (such as the splashers).
You will probably have to remove a fair amount of metal from the front bogie / cylinder area to allow curves to be negotiated.
Fitting electrical pickups to the tender will help the running no end.
If I were building another one, I would consider lightening the tender as much as possible - for example the cast tender top plate could be replaced with a piece of thin sheet metal (or even plasticard).
On the plus side, there's room for quite a large motor high up in the boiler. I used a Mashima 1628 with a High Level "Highflyer" gearbox and a brass flywheel.
If you want to portray a particular loco, consult the relevant Yeadon's register or similar reference. The K's model represents the locos fitted with a smokebox saddle (many weren't) and coupled to a tender with a cutaway at the front. The Ramsbottom safety valves seem to have been generally replaced by Ross pop ones in the early LNER years.
I have a 1983 vintage blow by blow account of how to build one of these kits; (including the K's exploded diagram). I can send you a copy if it's of interest to you.
Good luck with your build - looks very good so far.
Regards,
Alan