Good idea to run an external filter..... but....
One of the common failures we often see on the CD4E is poor cooler flow and subsequent internal hard part damage due to lack of lube flow.
The cooler return feeds the lube circuit so it MUST have good flow.
Here in Canada... the early CD4E trannys would fail from the fluid gelling in the cooler during cold weather. Fluid gels... no flow... tranny fails...
Ford "fixed" this by putting a thermal cooler bypass valve on the cooler lines near the trans. In cold weather the valve opens and allows unrestricted flow by bypassing the cooler until the trans warms up a bit.
The bypass valve is also pressure sensitive, so if the cooler plugs for whatever reason... the valve opens to allow flow.
We install an external cooler bypass valve on all CD4E vehicles that do not have them, to avoid repeat failure.
If your addition of an external filter causes restricted flow and you have a bypass valve...fluid will bypass the cooler = no or poor cooler flow = trans overheat in warm weather.
If there is no bypass valve and the external filter causes a restriction and poor cooler flow.... bye bye trans... the failure won't happen right away.... but some months later... We have often seen CD4E vehicles in the summer....that did not have a cooler bypass valve.... with hard part failure due to restricted cooler flow from the previous winter.
There is also an internal cooler/lube circuit control in the valve body, and we install one of these Sonnax kits in every CD4E we build.
Click on the part summary or instructions under resources to see what this kit is all about.
Be aware... to install this kit you also need to have the special reamers that are extra, and not included with the kit.
Does the Cougar have an external bypass valve fitted? Do you drive it in winter/cold weather?
If it will see cold weather operation, and doesn't already have one, a cooler bypass valve is strongly recommended!