Giesemann Fixture - Tridonic Ballasts

[mike31154]

I believe your assumption is correct. Any 250W ballast (electronic or tar), not to mention a dual is going to be much larger than that unit you have currently. Unlikely that it will fit into the fixture and even if it did, would certrainly generate additional heat which the fixture may not be rated for. External ballast(s) looks like the best option.