Eric,
The K-27 plans you mention were in the March and June 1973 issues of MR. I can't find the March issue, but the June issue has gorgeous 1:48 plans of the D&RGW 453 and RGS 461. Plans show both inboard and outboard piston-valve configurations, modern slide-valves, and extended smokebox. There are also loads of detail photographs. Fantastic!
The October 1979 MR had a set of plans for the K-37 class.
A set of Al Armitage K-37 plans originally published by Precision Scale Co. were reprinted in the July/Aug 97, Sept/Oct 97, Nov/Dec 97, and Jan/Feb 1998 Narrow Gauge & Short Line Gazette.
If you can't find back issues locally, I'd try Bob at the Trackside Emporium in Chama. He's dug up some great back issues for me (www.rollinstock.com).
Take care,
Jason Arnot
Calgary, AB