440 overheats once and then it's happy all day

OK, here............I'm not "into" all the ins and outs of this new fangled micro V and most especially marine reverse rotation, but make DAMN SURE the pump is rotating the correct rotation That is, coolant flow correct.

You can easily see this as the thing starts to warm up. Hold the throttle open some (hold RPM up), and watch the upper / lower hoses. The lower hose should show a bit of suck in, and the upper should show pressure.

Make sure the pump is ACTUALLY not broken loose from the shaft. I had that happen on a 426 once, and it acted a bit similar. This was in San Diego, and I had no heater. It did, however, run a bit hot in traffic, and wanted to puke after shutdown.

Make SURE it's actually getting full of coolant. To do that, pull whatever you can 'loose' when you fill it. Heater hose, the temp sender, etc, and fill "till you get water," then then fill on up from there.. The want the highest fitting you can remove under the T stat "coming up."

And drill some SIZEable holes in the stat ring to allow bypass let's say 3 or 4 1/8 holes around the outside if you have room.