A 255x70x15 tire it really tall, about 29" so it will help reduce your RPM at speed.
With 3.55:1 gears, 60 MPH would be about 2,500 RPM, and 70 MPH should be under 3,000 RPM (may vary a bit because of converter slippage and actual tire size.)
going from 3.55 to 3.23 is a 9% change, so your rpms would be around 250-300 RPM less.
Using a 255x60x15 tire (about 27" tall), and 3.55:1 gears, 70 MPH is about 3,150 RPM.