I doubt there's a direct conversion. The two systems have basically nothing in common.
Given that GURPS Traveller was written for GURPS 3e, if anywhere, the original GURPS Traveller book would have a system. Failing a system, it might be possible to reverse engineer what kind of framework the writers of GURPS Traveller used to convert Traveller ships to GURPS. I suspect the answer you'll find is "it was done by feel" though I could be wrong - it's been ages since I've looked at GURPS or GURPS Traveller.
I have the original GURPS Traveller book here. No conversion system.
There´s a ship design rule set in GURPS Traveller that seems to have been used to design the ships presented in the book - so, no conversion from classic Traveller rules.