Actually the X brace tubes are .875 x .049.....the short .50 tube(shoulder harness holder) is .035, all are 4130
The original diagonal is Piper part #10562-81, and calls for 7/8" x .035 1025. Not sure what Atlee calls for in the STC, but using 7/8" x .035 4130 will give you a 30-40% increase in strength by nature of the material alone over the original Piper engineering. Not sure where the .049 is coming from (again, unless that is what Atlee calls for)
For "slender" columns, that is correct. See "Slenderness Ratio" about halfway down in this page. https://www.engineeringtoolbox.com/euler-column-formula-d_1813.htmlColumn buckling strength of 1025 and 4130 is basically the same.