Looking back at the 4mm thread on RMW I'd say the kit designer has boobed, though no one's mentioned it, and from this photo you can see the solebars protrude below the headstock. IIRC the solebar should be 9" deep (can Paul confirm?) so a quick measure and you'd soon know if the solebar is too deep or the headstock not deep enough and adjust accordingly.
Bit of a faff either way though for what should be a straightforward build
Umm I won't comment on etch kits, I'm sure there are some that are accurate, but I think you'll find far more accuracy from the plastic kit manufacturer's in 7mm, although they tend to use generic underframes which can mean some of the plates and bolts are in the wrong places.
Was it on here that Exactoscale 7mm was mentioned? I was really surprised to see they offered components for 9 in and 12 in solebars. When we were measuring I have a recollection of us finding an elderly wagon with a 9in solebar and being surprised, but 10 in. is the usual standard, and there are 12in. solebars. It was something that went the rounds in my earliest modelling days that solebars were 9in, but as I say something that I believe ended a long time ago. 9 in. is in use for a steel frame on the 1906 RCH minerals of 8, 10 and 12ton but not for a steel frame on a 15 tonner of 1903 - see in your copy of
Watts, A J (1998) Private owner wagons from the Ince Waggon & Ironworks Co. Publ Historical Model Railway Society. 190 pages ISBN 0 902 835 25 4
I don't know much about this manufacturer, but I suspect they are used to elderly prototypes which are accurate for a 9in, solebar
Paul Bartlett