8 inch is the only way to go.
I have to street cars with 9.5 convertors to start with then switched to an 8 inch and in both cases I like the 8 inch better.
As far as an 8 inch making too much heat that just is not true as long as you have a large cooler.
This year I drove my Challenger round trip to the MATs from PHx AZ and the trans fluid never became to warm.
The out side temp for the drive up there was around 90 degrees and about the same on the way home.
My Challenger weights 3700 lbs with me in it and only has a 3.55 gear.