Unfamilier with the downtube shifters, but you have a variety of other options from Shimano

Bar end shifters, SL-BS78 http://techdocs.shimano.com/media/te...9830728256.pdf

rapidfire+ shifters which are road compatible, these are non series, like the R770's, the problem you may find with these, is that their handlebar diamiter won't match the bars you want http://techdocs.shimano.com/media/te...9830677318.pdf