Skip,
I am a big believer in the type of cooling system that you are building as it does not require the water tank to be a pressure vessel. One thing about water to water heat transfer is that it is much more efficient than air to water and because of this the actual radiator size can be much smaller which would give you more water volume in your tank. The real point gets down to how much water you have in your tank, what is its starting temperature and how much heat are you going to transfer into the water in a single run. From this you can calculate what the water temp will be at the end of the run. If it is over 212 deg F (at sea level, lower at altitude) then your tank needs to be bigger or the water colder. This is one of the reasons to use ice in the tank as ice has a requirement for additional energy when it converts from solid to liquid (energy of transformation) so you actually can have a somewhat smaller water tank. I would also recommend that you use a thermostat as the high differential temperature between the water/ice in the tank and the water in the engine/radiator will never allow the engine to warm up and when it finally gets to operating temp the water in the tank will also be at that temp and now you don't have any differential temp reserve for making the run. I would also suggest using the BMW 320 three way thermostat as it is set up to recirculate the water in the engine at full pump flow ( a normal thermostat simply prevents any water flow through the engine until it opens) and then it modulates the water flow to the radiator to maintain its set temperature. Much better set up and they are cheap, around $20.
I would suggest some sort of recirculation pump in the tank to keep the tank water moving through and around the radiator matrix along with some sort of baffling that encourages the tank water to flow through the radiator and at a pretty good rate. Don't forget to use a big water pump on your engine, over 60 gpm if possible, velocity through the radiator core is what you are looking for the more the better.
Love your build and am really looking forward to seeing you at Elmo or B'ville.
Rex