Well, the new HX arrived and is installed, but there were a couple of "gotcha"s along the way. Monitor has made some changes in their design in the 8 years since I installed my original HX:
1) The zinc location has changed from the bottom to the top, on the diametrically opposite side. In my case, this moved it from 100% accessible to a cramped and blind location.
. But it's better positioned to protect the raw water barbs.
2) The coolant return fitting location has moved from the side of the header tank to the end. Fortunately my hose had enough slack to reach.
3) Both of the coolant fittings have changed from 3/8" NPT to 1/2" NPT! This caused me some grief as I had to hike all over Charleston looking for new elbow barb fittings. Never found them, but did find some adapter bushings that allowed me to use my original fittings.
Never a dull moment...