1970 c20 transmission

[1970c20owner]

I have a 1970 c20 camper special with a 350 with a four barrel holley carb. My automatic transmission developed a problem and I am looking into maybe upgrading it. I was told that I can install a 700r4 instead of the th400 that it currently has. I would like to be able to drive on the freeway without having my engine pulling high rpms, but I also have s 30' travel trailer that I want to be able to still tow. Anyone out there has any suggestions on how I should proceed?
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