I am going to try and mount both the battery and the DC-DC converter in the standard battery position.
The first problem I had to face was that the DC-DC converter closely resembles a computer power supply. So the last thing I wanted was for dirt or water to be sucked through the unit.

My first idea was to buy an airtight food container and mount the DC-DC converter (and a few other things) inside ... Here is the container I picked up from K-Mart ... It is slightly larger than the DC-DC converter.
I mounted the DC-DC on one side leaving an airgap below
I drilled out another side of the container for air intake and made a foam filter
I installed a 12V extraction fan on the lid
I installed the 50A shunt for the low voltage ammeter
I installed 2x 12V relays with their own fuses (for the power steering pump and vacuum pump)
I installed a 40A DC circuit breaker for all the cars remaining 12V power.