That is a great de-rusting technique. I'll have to remember that one. Could be useful for many things.
I'm not an expert on the E-Type, but I wouldn't change the thermostat for that reason. If it is designed to run with that value, I personally wouldn't go any higher, even for winter, and especially just for getting more heat. Have you checked the heater core? It could be partially blocked, no allowing enough coolant to flow through. If it is clean, you may try the trucker technique. You can cover part of the radiator with cardboard and WATCH YOUR GAUGE!!! That way you only need it when she is in the car or on warmer days. Since it is a 1970, I'm guessing it is a 6. If it is a 6, just use a square about 12x12 to start with. Go bigger or smaller to adjust temp...and again WATCH YOUR GAUGE!!! For someone with a 12 reading this it may have the dual input, I'm not sure if the V12 E-Type has the two input radiator like the XJS or not. If it does, be very careful which part of the radiator you cover up. You could overheat one bank leaving the other cool. I can't imagine how it would run doing that. But you probably wouldn't have that issue with a 12 anyway. Those are a battle to keep cool.
I'm grinning while I type this one, but you could get her sleeping bag to ride in. If it is a "polar" one with the hoodie attached, hey, you may even be able to put the top down.
Anybody have any other suggestions...coments?