Though you have the bosses for downtube shifters, you may well find that bar-end shifters are more convenient / easier to use. I have bar-ends on an old 531 frame (Dia-Compe friction) and they work well. Friction is no problem to get used to and it gives you the option of using wheels with different speed cassettes, which I found handy for commuting when a wheel swap could be required at short notice.