I would suggest asking this in the Mechanic's forum if you haven't already. I have DA bar end shifters on 4 bikes and just recently re-cabled a couple sets, which means that I just had them apart. Going from memory here since I haven't installed a pair of down tube shifters in a long time, I doubt that it would work. I would certainly want verification from someone who has done it before I ordered levers.
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Keep the chain tight!