Adding a transmission cooler

The only issue is a built in screen that's clogged or some other restriction in the OEM cooler. In that case flow would be the same in both.
My trans guy always recommends a new cooler to replace the OEM cooler, not in addition to it. That is in the different brand, newer, front wheel drive, whatever. Although I haven't yet, I do believe if I took my 67 904 to him, he would say the same thing.
If you prove you have good flow through the OEM cooler,,, is additional cooler even needed?