Fit oil cooler after fitting cowl
I used a James cowl and a 4" prop extension, a combination that "squeezes" the cheeks down to a smaller height toward the back of the cowl. Fitting the oil cooler behind the #3 cylinder required some very careful fitting. The moral of the story is to consider fitting the cowl before permanently installing the oil cooler, or you may run into clearance problems.
For what it's worth, my cheeks didnt have enough space to mount the cooler vertically. I mounted my oil cooler to the engine mount behind the cylinder, with the face of the oil cooler about 15 degrees from horizontal (not vertical). A carefully formed rubber duct directs air from a cutout in the rear cylinder baffle to the face of the oil cooler, and a homemade fiberglass exhaust duct directs airflow from the oil cooler down toward the cowl outlet. Works great on my O-320. Requires partial blocking most of the year to achieve 180 degrees.