Lsrjunkie,
If you are looking at 400 hp you should have about 40 gpm (in your post you say 60 gph, but I am sure you meant 60 gpm) so if you have a "real" 60 gpm you should be good. The -16 lines should be fine, if you are going with a water tank I would suggest using some sort of pressure vessel so that you can run the cooling system at 17-25 psi pressure as it has a huge effect on keeping your engine cool. If you go the "radiator in the tank" method remember that water to water coolers are 7-10 times more efficient that water to air so you don't need a very big radiator but you must provide a pumping system to move the tank water through the radiator to be efficient. As far as warming up the engine I highly recommend using the BMW 320I three way thermostat from around the 1983-4 year. This thermostat will circulate the water in the engine through the engine, not back to your tank, until the water is warm and then it will modulate the water from the tank to the engine to maintain its setting temperature. Also remember that electric water pumps don't like to make pressure so any kind of restriction greatly reduces their flow.
Rex