Whew! I would have to say that sizing spindle lengths for square-taper cranksets is one of the darkest of arts, respecting the outfitting of vintage bikes. The original fit, "standard"used and tolerances of manufacture combined with the low angle of taper (2 degrees), how "thick" the bearing cups are (Campagnolo made both "thick" and "thin" at different times) crankarm socket wear considerations for previously used cranks, whether any lubricant is used, and how tightly the cranks are torqued, make it an extremely "trial and error" proposition. Oh, and did I mention that some spindles are slightly asymmetrical, the crank end being a bit longer than the non-drive end (see Campagnolo cuts above)? Then there's the conundrum of "Campagnolo" taper. Campy tapers were actually somewhat stouter (or larger in dimensions) pre-1993 or so, when Phil Wood did not really manufacture a "Campy" taper, but told folks to use the JIS taper -- after that, Campy slimmed their taper a bit to match ISO taper. Lots of fun there. Phil Wood themselves sort of acknowledge the problematic nature of all this on their square taper page:
"Phil Wood & Co. does not assist in determining which bottom bracket is correct for any particular application. We recommend taking the frame and crank to a local bicycle shop as they should be able to help in selecting the correct bottom bracket that is needed."
Amen.
A useful point of departure would be to get a matching crank and spindle that were originally intended to be used together for a double crankset (and the correct size of bearing balls). Presumably that would give a reasonable chainline (where the midpoint between faces of the two chainrings is located about 43.5 mm* from frame (and presumably the bottom bracket shell) centerline. Failing that, I've found that Campagnolo and Sugino tapers and spindles are fairly interchangeable. Stronglight and T.A. cranks seem to work well enough with JIS taper spindles too. Sticking with JIS for (now only "vintage" where Shimano is concerned) square taper bottom brackets and cranks might be easier, actually, than getting into the weeds with the more C&V European stuff might make things easier, in the long run.
But if you're mixing and matching, trying to determine in advance "what length spindle do I need" is just a crapshoot until you actually try it. I would suggest getting access to Sutherland's Manual 6th edition (apparently available in PDF form online), which has a very good section on bottom brackets dimensioning and compatibility between brands.
What you're looking for are two things basically: achieving a decent chainline, and getting the fit of spindle taper and crank socket sufficiently correct so that the end of the spindle is neither recessed too far into the socket (inadequate support between spindle taper and crank) or projecting out of the socket (can't tighten the crank arm sufficiently). Having the spindle end (when torqued down!) about 2–3 mm from the end of the socket is ideal.
At the very least, make a decision about exactly what cranks you actually want to use, and then people here can be more helpful regarding what has worked for them in that situation.
*43.5 mm is not a hard-and-fast criterion, of course, just the general neighborhood of where a double crank wants to find itself. A prime consideration is whether there's interference (free-spinning, or under pedaling/chain load) between the inner ring and the right chain stay; which also has to do with how large in diameter the inner ring is.