Get the tranny rebuilt by a good tranny shop. Tell that you plan on towing the trailer. They can beef it up a little and give you firmer shifting. Also get the biggest tranny cooler you can. Heat is the worst enemy of auto trannies. The cooler you keep it the longer it will last.

BTW, you would have to go up to the 351 if you were to swap engines. The 302 would not give you any towing gains over the I6. The 300 makes as much torque as the 302, but has a flatter torque curve. Also, keep out of overdrive when driving in the hills.

Sven