I can't believe this thread has been on here for three days, without someone with the experience or knowledge to answer the questions, and I have spent the time doing the thread search, if the answers are on this site, I haven't found them.
So here are the answers, no there is not a water cutoff to the heater core, other than the thermostat, yes it is a constant flow through the heater core, and yes on the older trucks, you better take the time to do the flush. "back flush" the heater core, and clamp off the heater lines when you are flushing the rest of the system. I spent more time searching on here, far more than it took to do the actual work, and I found more support on other sites.
I have been totally amazed, so I hope someone else finds benefits from this.