Do you have any way to monitor trans temps? I can program it as an X gauge for my Escort. I had my lower grill blocked during winter. The first warm day I drove it like that I noticed coolant temps were 5-10* higher than normal. But trans temps on the highway were 30-40* hotter than normal. So basically I think you are right to be concerned about how this heat is affecting your transmission, and I would definitely add more airflow and consider putting the stock thermostat back in.
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