I finally have some progress on this project. I bought a set of digital calipers and a dremel 4300 kit and used that to reduce the size to 14.8mm in order to fit in my heater core pipes. I started using the dremel kit with the sanding table attachment and the metal sanding wheel which didn't work very well on aluminum so I switched to the cutting bit and kept the sanding table which enabled me to slowly take off a small horizontal cut each pass and then rotate the insert and repeat another small cut. It was a slow process of taking off a small amount and then testing the fit but finally got it done. Put it all together with the red RTV permatex and it seems to be holding pressure. My problem now is that the hole for the air bleeding is leaking small drops of coolant under pressure after shutdown, so I have added another clamp on the rubber hose in hope to stop that. If that doesn't keep a tight fit I will have to go get some proper spring clamps or just rtv that hole as in my experience air bubbles usually will bleed back to the coolant reservoir under normal driving or worse case by raising it up higher than the heater core once the thermostat opens.

As other posters mentioned if you pull off the rubber boot at the firewall it makes it much easier to see what is going on and there is plenty of plastic tube extending in to the top of the heater core. I wish I did that first as gives much more room to work. You can also see the arrows on the tubes showing flow direction. The drivers side is inlet (right side when standing looking toward the engine with hood open) and the passenger side is the return from heater core back to the engine compartment.