Transmission cooler question
 

I have a 91 ranger auto trans, this summer I was reading tsb and it stated that the trans cooler on this year may be to small, I was already having tranny problems, so I replaced it with one at least twice as large,( it was the only one they had) Now I had also read that it might be a good idea to cover half of this large cooler in the winter because the fluid might get to cold and thicken causing transmission problems. Keeping in mind I have a clutch fan that runs all the time. I called one auto parts store and asked this question, and he did'nt think I woud have to block any of the cooler, I just thought I better be safe than sorry and ask this question here as well.