It is possible that the B-7 was the last to go, but, it was rebuilt in 63 if I'm not mistaken, and that is when the 330 and 331 were built, so it is concevible that the steel coaches arrived first.
On a side note, I have heard that these first two steel coaches had there trucks come from RPO cars 66 and 122. The 122 is at Ft. Lupton(without original trucks reportedly) and the 66 at hermosa, but I thought that it looked like the 66 still had trucks. Are they replacements? It would seem odd that the Rio Grande would have spent the money or time putting other trucks on, but, then again, they weren't known for repeating procedures.