Was yours one that came with a 60" base, or the optional 80" base? There's a big difference in the casting for the wider ones.

For the original question, I don't think it matters what order the spacers are put on. My gut says wider spacer to the inside, but I'm neither an engineer nor metal man. It will be the axle housing that fails like you were told, and I'm "quite" sure that would only happen on a shock load such as hopping a railroad track at an unsuitable speed.

I once had a Cat engineer tell me that the number one thing they were afraid of was being the engineer that designed a frame that failed. Their calculations had some serious safety factors built in.