I've ready to move away from cup and cone bbs and make the leap into the cartridge type, at least on this particular bike.
How do I decide what length to get? The spindle that's on it is too long, almost certainly, at 119mm. Something I grabbed out of the parts bin. The crank set is Shimano FC 1050, double, and the bike is a '79 Trek 930.
TIA

One thing to keep in mind is that most of these modern cartridge BB's are not offset. Unlike most classic cup-and-cone BB's, the modern ones are is usually the same on both sides - meaning you can get a good idea of where the spindle will land by dividing the length by two and comparing that with a caliper against the BB and existing taper.

-Kurt

According to velobase, you should use 113mm if the Trek has an English BB-shell, or 115mm if it´s Italian. I´m using 115mm symmetrical with 6400 (The are specied to use the same length of bb) cranks on both my Olmo and my Fondriest, and thats a perfect fit on both.

So the spindle length has to do with the crank set. That makes sense. Rivendell has them in 110mm, for $20. Maybe I could do it with a different crank set.

Correct. Different cranks require different bb´s. You could no dubt find a crankset that will fit the Rivendell bb. But why would you want to do that?. If the 105 cranks you have now are in good condition, and the chainwheels are not worn out, then it´s a perfectly fine crankset. I´m convinced that you can find a suitable bb for the same amount or less, than what the Rivendell bb + a different crankset will cost.