Man, I've seen some of those RD400 tanks on CL recently, so if the ads are still posted, I'll try to give you some leads...
Check back here frequently over the next several days; if I post a Craigslist link, you'll have to act fast before the tank is sold.
Let me know what part of the world you're in, so I can try to help you find one relatively local to you.
Here's a link to a .GIF of the tank used on the early XS360; last year for this tank was 1977:
http://images.powersportsnetwork.com/fiche/images/Yamaha/1977/Motorcycles/47_fueltank.gif
IIRC, most (maybe ALL) of the XS lineup (XS400, XS500, etc.) of that era (ending in 1977) used the same (or a VERY similar) tank, along with the RD lineup. With a little luck, you might be looking at some links soon, so be prepared to act quickly.
EDIT: Speak of the devil: 1979 RD400F Daytona tank on CL in Los Angeles:
http://losangeles.craigslist.org/sfv/mcy/743806870.html
NO photos, but there is a phone number; in the meantime, I'll keep looking... -- kd
Again, let me know your general area, so I can focus on what is close to you. That way, you might be able to see it in person before you buy; even if it isn't close enough to pick up personally, you should be able to avoid heavy shipping charges.
-- kd