Did you already punch the holes for the elements? You may find you have more flexibility of where to mount it by going with a single gang box vs. a double gang box. My method - I mounted the dip tube firs. I then spent some time looking at the tube, the shape of the element, etc. I put the hole for the element just slightly above the hole for the dip tube (measured center-to-center), and because of the way the element curves, it actually deflects 'around' the dip tube. I don't have a False Bottom in my BK to deal with, so your results may varry.
Once I had the element installed, I measured for the HERMS etc. The other thing I did to help with element placement was to add 5 gallons of water to the BK and measure the height - 5 gallons is the lowest volume I would ever expect to have in there, and I made sure my element, temp probe, and other fittings were below that level (including my return line for recirculating).
If you have other questions on how to drill, where I put things, or anything like that - don't hesitate to hit me up. I just finished my setup, so the memories are fresh.
-Kevin