There's no real conversion, as dTon is really a measure of volume while the Spaceships rules revolve entirely around the ships mass. The best conversion I've done is to just use a rough approximation of the ships Loaded Mass from Traveller (LMass) as the basis of the ships mass in the Spaceships design system, then set SM accordingly based on the Hull Size Table on Spaceships p. 9.
RPK has rules for using
GURPS Spaceships in Traveller.
I have some
very rough guidelines for intermediate SM ships in GURPS Spaceships, as well as implementation of those rules and RPK's rules in my
GURPS Spaceships Design Spreadsheet.
Using those I've designed 100dTon versions of the Suliman as being SM+7 and 300 tons mass while the 200dTon Beowulf and Empress Marava are SM+7.5 and 600 tons mass.