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It won't take 300 hours, the crank is made from aluminum, they're easy to files down, maybe 20 minutes, half an hour max (I did this for a customer bike who have similar problem).
It will be especially quicker if your files isn't too badly worn.
edscoble-
The only file I have is only a bit bigger than a nail file. Last used it to file the wooden bridge on my double bass to fit a pickup, I think.
Anyway, after several hours of filing it's working. Gonna nip out and buy some proper files tomorrow and get them flush, but the spider just about clears the chainstay when folded now.
I have now noticed that the slightly longer crank means that if I spin the cranks after tucking the rear wheel under, the end of the right crank will just about clonk the nearest part of the rear derailleur.
Not really a huge issue, but something worth bearing in mind if you're changing cranks and have a choice of different lengths.
TurtleRecall
I do have a file somewhere. I'll have a go and see if it looks like it'll be 300 hours work.