A 4L80E would be a wiser choice than keeping the TH400.
You will regret dumbing down the rear gearing. If I could do it over I'd have put 4.56s in the back of my wagon with the OD trans. That's the long term goal anyway.
Put a 275/60R15 on the back.
5664s up front and 5413s out back for a sporty ride.