That sounds like you have done your homework!  here are a few thoughts I have;  the dirt you want to remove is also a thermal mass and has much more potential because it also exchanges with the surrounding soil and since you have a warm dirt ceiling as a floor, more heat in the soil wants to rise where the soil is not frozen... and if you are going to bury the 55 gal drums directly (uninsulated)  I dont think you will get a lot for the effort. if you were to insulate them then they become heat storage, and in that purpose rocks hold heat better and do not have dirunal heat respiration.  there are a million ways to use both also.   depending on what kind of fish and tanks you use along with sun exposure, you may not need much heat storage .?.  insulated walls are a plus, mounds even better, but a shared wall with a heated structure takes the cake.  I built a 14x16'  greenhouse in a hurry with an early coldsnap bearing down and am begining to wish I had twice the space for more growbeds, solar heater,woodfired heater, heat exchangers, plumbing and room to work. 

I'm giving up on my tillapia growing this winter but they will stay alive and my garden is growing verry well on 55 cents a day in electricity until I find some space for a stove of one design or another.

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I'm doing research and design in my "lab"  and  would love to see/hear your solutions to our climate 3714943