I think you're on the right track. I'm not a sour fan, but souring the mash and then boiling sounds like it would fit your needs perfectly.
A 24 hour mash may be about right for this, depending on how sour you want the finished beer. Twenty four hours might be just a bit "tart" and not really sour.
Another thing you could do is add lactic acid to the wort/beer if it's not sour enough from the sour mash. That can be done more to taste, and you won't risk overdoing it.
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