I'm in a somewhat similar situation, and here's something I've considered: continue to drive the SD for a few more years. If I fail to stop, or do not get around to stopping/fixing the rust, go down south and get another SD without rust and transfer some mechanicals. This is partially because I have rebuilt my engine and I want to keep at least that. But not having a garage really limits your options, so I would keep driving it, see what things you could use like POR-15 to stop the structural rust.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.