Drat! Nearly right. The Thunder Panser was a later development published in November 1975 RC Modeler.

I checked the May 1972 MAN but the span is not mentioned in the article.

I have scaled up many plans using spinner or wheel dimensions as a "scale" and it works fine. You have a 1" dimension as dihedral and a 2" nose wheel on the D-20 plan and I bet the distance between the foam wing root and tip templates is 30".

Ray