I've seen this happen a few times, and had it happen on my hotrod Skylark when I failed to seal the (used) 4 BBL intake properly when I installed it. First figure out where the leak is, and fix it. Drain all the oil (done), change the filter, fill with the proper level of oil, and run it without a load. Then change oil and filter and run the same way. It should only take a couple of changes before the coolant is flushed out. Let the pan drain awhile, because the coolant/water mix is heavier than the oil. If coolant has gotten into the exhaust pipes, it's going to blow water vapor for many miles. AMHIKT.
