The entire carbon based global warming panic is based upon computer modeling. The projections have been used for nothing but Gov. subsidy and grant schemes. The stated goals of the Green energy movement is to lessen the carbon footprint so as to lessen climate change. The simple fact is the predictions as to the effects of fossil fuels and climate change is NOT based upon any empirical evidence. There is no proof that the increase of CO2 in the atmosphere is cuased by human activity. There is no proof that the increase in CO2 in the atmosphere causes increase in temperature. I do not believe that Obama is interested in reducing the carbon footprint he so urgently desires. If he were he could allow natural Gas exploration on federal land. But he does not want to bring down the cost of energy or "clean up the environment" He wants the demise of capitalism. And he will do it by any means possible. That is in fact his stated goal. This whole climate change goal is not driven by anything other than Obama's political and social adgenda. He really does want to fundamentally change the USA. And he is on a path to do exactly that.

I'd probably up the ISO a bit, and start with some shorter exposures. Otherwise, *film is cheap*!, and the only thing you have to loose is a bit of time! but to gain, a lot of knowledge and experience! With longer exposures like you are thinking, you may want to try some in camera noise reduction......unless you have software to do that later. Especially if it is warmer, the camera can get quite noisy. But mostly, just have fun!
You may find that that 300mm is enough. I haven't tried it myself yet. A wider F stop may be an advantage if you can go there, though conventional wisdom suggests closing it a stop or two for better sharpness....