8" converter on the highway ??

DITTO.
Unless you cruise in the non-slippage rpm of the converter, you will generate heat.
Determine the gear you will have to run. And it will not be a MPG gear.
Right. That's the whole key. Staying out of the slip.....but with modern converters that's REAL easy to do since they don't slip. lol I found zero need for an additional cooler.