You wrote: Le't's start by cleaning up a few misconceptions, before getting to the point -- which is a very good one.
"Low and slow" is a colloquial term without a universally definition; but it's almost always hotter than 210F. Cooking at temperatures below the boiling point of water is another thing altogether. It's also not particularly safe, and I can't recommend it.
Cooking BB ribs low and slow typically takes about 4 hours, and spares take about 6.
Most charcoal and wood-fired smokers have a relatively restricted range of temperatures which they can hold steadily. Usually the range is within a few degrees of one or both sides of the 215 - 235F range. It's the rare smoker that can hold a steady 325F.
You can smoke a standing or prime rib to 140F internal at a low and slow 225 - 250F, and do a pretty good job. However, since the roast won't absorb much smoke after a couple of hours, you might just as well transfer it to the kitchen oven and finish it at a higher temperature for a quicker cook and a better crust (aka bark).
If you're interested in more specificity, let me know what kind of smoker you have, your experience level with it, and anything else you think might be relevant, and we can start getting specific.