I agree that a factor of 2,7 seems reasonable.
21 in gun (experimental German 53 cm) are 335 tonnes
http://www.navweaps.com/Weapons/WNGER_21-52_gerat36.php
20 in gun (never built Japanese 51 cm) are 227 tonnes
http://www.navweaps.com/Weapons/WNJAP_20-45.php
With the factor of 2.7 gives 613...