For what its worth, my Father was a trans rebuilder. I used to help him in his shop from age 9 to 23 (he retired 4 years ago when my Mom got alzheimers but still rebuilds the occasional trans)

The 4L60E is a reliable unit for the most part but is old technology. I change my trans fluid and filter every 15,000 miles period. I installed an extra magnet in the pan. I have 118,000 miles on it and it still shifts out and goes into TC lockup like the day I bought it. An external transmission cooler is an excellent idea for the 4L60E - you will definately extend the life and performance with a cooler.