You could always set your model T's output inpedence to 8 ohms. Then, use one input out of the amp to a 412L (8 ohm impedence), and then use the second amp speaker output for the marshall cab, going into 16 ohm mono. Going from 8 ohms out of the amp into 16 ohms cabinet input impedence should be safe , but may cut the volume a bit. If you went from 16 ohms out of the amp to a 4 ohm cab, then you would have a problem.
You can't really match impedence with the 412L and the 1960A. There is no 8 ohm option for all 4 speakers, and if you're only going to use 2, like Isaac said, they will be getting twice as much power, and might not be able to handle it. Also, The G12T-75s in the marshall cab have a relatively low speaker sensitivity, especially compared to the 412L speakers. This will factor into the volume disparity.
There is always the third option, which I would recommend over the others: run both 412Ls out of one model T set to 8 ohms, and use the other model t for the Marshall cab. This would yield the most control for volume balance between all the cabs, and all impedences are perfectly matched and safe.