I would just fill the radiator with straight water, then add some if needed. Then after the engine has been run for (cam break-in), drain all the water and then refill with water and antifreeze. The reason for this is if you have a leak during the first start up, you only deal with water leaking out and you don't waste the antifreeze. This is what I did with my 340, You can leave the thermostat in too. Be sure your fuel lines are sealed up good and don't leak, it is nice to keep it running for the first 20 mins and not have to shut it down. Also, make sure you have a fire extingusher and a extra helper with to watch for any problems if you have to sit in the car while it is breaking in.