The Main was built by Mackie & Thompson in 1904. The company were taken over in 1912 by Harland & Wolff, and any records which still survive may be in their archives in Belfast. I had a look at what Harland & Wolff holds and couldn't find anything else mentioned other than the takeover, but it's possible there is something on the Main. I don't know where the archives are held but you should be able to find out easily enough. As the ship was built in Scotland, two other sources you might try are Glasgow University Archives and Glasgow Museum of Transport.