Thread resurrection. I just installed a setup just like this on my 99 grand marquis. I had the same issue as you. Original green coolant left in way too long, system nasty. I flushed the system 3 times in total, last time replacing all of the hoses since they were 20 years old and lined with scale/rust. I flushed the block very good since all hoses were off and even put a new reservoir on. I came across the picture and started laughing, as I installed the filter in exactly the same place you did. However instead of Teeing into both like you did I just eliminated the heater core to water pump return tube hose and ran a piece of hose from the outlet of the core to the filter, and filter outlet to the water pump tube. I'm using a B5134 baldwin and it has the restrictor. Before I did this work I had 160-170 degree vent temps and now only get 130-140. at best I flushed the core again today and it came out clear and made sure I had no bubbles in the air system. Suppose the restrictor in the filter is causing the coolant to dwell in the heater core too long, causing it to cool off more than usual? I know normally one would install the filter BEFORE the heater core instead of after but I didn't want to mess with the plastic heater nipple off the intake more than once. So I installed it after to avoid future chances to break that thing off.