No pics yet - planned for a future VBQ.
But it really is quite simple. Here you go: Put masking tape on the front of the spider arms, so they don't get scratched. Then use a small round file (chainsaw sharpening file works great) and file into the cracks, until they are gone. Removing 5 mm or even more in this spot should not affect the strength of the cranks - you are removing a stress riser, so you are making the cranks stronger.
For most of this filing, you are removing material from the spider, not the attachment of the arm to the spindle. And I bet you could almost remove the two spider arms without the chainring collapsing on you. If you have to file so far that less material remains on the crankarms socket than there is on the left (other side) crank arm, I'd retire the cranks instead of trying to salvage them! The same if your cracks run into the arm itself.
The masking tape is important, as you will scratch the surface otherwise. Of course, no guarantees whether the cranks are safe to use afterward.
Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
c/o Il Vecchio Bicycles
140 Lakeside Ave, Ste. C
Seattle WA 98122
http://www.vintagebicyclepress.com