Steve,
I switched to a heat exchanger type setup last year and an ice tank after reading through the posts you referenced earlier. What I ended up doing was having a custom aluminum radiator made that the core is enclosed to have water flowing across rather than air. The radiator core size is only 15" wide x 5.5" tall x 2.75" thick. I have an ice water tank which will hold about 9 gallons of all water (or about 40lbs of ice + water) and has it's own electric pump to circulate across the radiator core. I have this circulating pump set up that I can manually turn it on or it can be automatically turned on by a trigger point from the engine water temperature gauge, serving as an electronic thermostat.
I'm running Flathead Ford which is not at the 300hp your talking of, but it dumps a lot more heat into the cooling system compared to that new fancy overhead valve stuff, so probably a decent comparison. This year at Bonneville with the engine starting temp around 110-120 the ice tank would begin to circulate right around the 2 mile and maintain whatever set point temp I had it at (usually 180-190). Once on the return road and turning all the pumps on it would cool the engine down to about 140. The really nice thing was we could take out some warm water from the ice tank, put in a bag or two of ice, run the pumps and it would be right back at 100 degrees and ready to run again. I am really satisfied with the setup I ended up with, which will have enough cooling capacity to an eventual goal to be able to run the long coarse, if I can make enough horsepower.
When I was considering the change to this system I also looked at heat exchangers and I stayed away from them mostly because of space constraints. However if you do go that route pay attention to their BTU transfer rate at different fluid temps. A lot of these are intended for much higher temperature differentials than you will be dealing with and can be misleading when looking at the numbers.
In the pic below the tank at the back of the engine is the "radiator/heat exchanger", the one with the cap off is the ice tank. The engine runs fully pressurized and the ice tank is vented and the water could boil off if need be. I agree with Rex and would avoid a fully unpressurized system for the localized boiling.
Untitled by
Andrew Welker, on Flickr