Here's what I'd do:
First, I'd measure from the center of the BB shell to exactly 160mm up the chainstays, to see where the crankarms would theoretically contact the stays. Measure that up, from the outer edge of one stay to the other, at the 160mm from center-of-BB. This would be your minimum tread; add a few MM to allow for clearance.
Next, take both crankarms and mock them up on your widest current spindle, free from the frame. No need to worry about bearings or cups or whatever; you don't even want pins b/c you'll want to have the crankarms running parallel to eachother, with the pedal-ends directly across from each other and the ends of the spindle flush with the outer surface of the crankarms on each side. Measure the length between the inner surface of each crankarm at the pedal-end; this will give you the tread for your cranks with the current BB spindle.
Finally, take the arms off the spindle, and measure all three lengths; the space between the cones' "shoulders", and the space between the cones and the end of the spindle on each side. From here, you should be able to extrapolate how much more length you'll need at either side, and you can probably estimate whether or not the 152mm spindle will do the trick; if not, it will aid you in your quest to find one that does.
Throughout this whole exercise, you can also find out what your approximate chainline is with the current spindle (measure from the center of the inner segment of the spindle to where the sprocket's teeth intersect with the spindle); this will get you within a mm or so of the actual chainline of the current set-up. However much longer the new spindle's drive-side segment is, that's how much further outboard your chainline will become with the theoretical new set up. I imagine you'll be increasing it by at least 5mm, based on your post above. That's a significant difference, but we can almost certainly find a way to work around it at the rear, depending on what your rear hub and sprocket(s) look(s) like...
HTH.
-Rob
ps- LEt me know if you need pictures of any of this; i have a crank/spindle i can demonstrate wth, plus some bare frames, but i don't have access to it all now, b/c i'm not at home. When i get off of work tonight, i can hook it up.
