I just installed a new ECP radiator in my 305 86 Monte LS. I've been flushing the coolant from the system and it's running pretty clear. My question is this: if I haven't drained the coolant from the engine block by loosening the bolt/s on the side/s of the block, I assume there is currently just water in the system. How much antifreeze should I add to the rad/system to make sure I end up with a 50/50 mix? I'm trying to do this without fighting with those engine bolts as I assume that makes things a lot tougher...