put your heater (or 2 smaller ones) near the inlet and outlet of the filter - perhaps on the side pieces of glass.
that should solve the heater problem.
as for the size difference of the backgrounds - either position them in such a way that they do not look "funny" in the tank either by minimising the space at the bottom or the top. id say if its 10cm short, then put the background about 3cm from the top and 7from the bottom. then you can put a crap load of plants and gravel, Real Rocks (like big, flat ones) at the bottom.
i have a picture in my head, i wish i could transfer it on to here, lol