If your using RSRDC you have a good chance of finding the Y in Truk...Without RSRDC Truk doesn't really exist in game.
With Stock or Stock W/RSRDC it should only take 6 Mk14's to sink it, with a Supermod it takes about 10 Mk14's or 5-6 Mk16's. I wouldn't even try sinking it with Mk18's.
To find them on the move there's always 2 Yamato's in the Center force during the Battle of Leyte Gulf. With RSRDC near the end of the war it's constantly coming out of the Bungo Straight.