Just as a side note, I restored a somewhat similar 259 recently. When I had the loco fully disassembled, I was amazed at how weak and flimsy the frame was in it's mid-length area, in way of where the boiler shell side panels tuck in. Not much meat designed into the frame there, and as such, it had poor bending and torsional strength.
I feel your pain!
Good luck,
TJ
IMPORTANT -- If I recall from my Doyle book, there are TWO completely different types of 258's. One is a near-clone to the 259, and the other is quite different. So, if you shop for 258 parts, make sure you understand which loco type you're hunting for.