The other issue is that even with base and mast tip extensions, you will be at the exact luff length. Will you be able to downhaul the appropriate amount? I guess it depends on how precise the measurements are and the pulley location.
How tall are you? If you extend the mast up 57cm, your boom my be below the reinforced section of the mast and you my crush the mast when clamping on the boom.
A three piece mast sounds like the solution.
At one time, I had a mast locking system which would work with the bottom section of the mast. A gizmo plugged into the base, twisted on tight and had a cable locking system which attached to the roof rack.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.