Here is for the speedometer gear calculator.
http://www.tciauto.com/Products/Tech...eedo_gears.asp
Here is a tire size calculator.
http://www.miata.net/garage/tirecalc.html
Here is another link to check out.
http://www.ringpinion.com/Calc_RPM.aspx
I think 4.10s would be fine for the 3/4 ton. Unless you find a 14 bolt ff that has 4.56 gears. The 3.73 4.10 gear ratios are pretty common. Having the 700r4 is going to get you off the line a little better vs other automatics.
I would think about running an external transmission oil cooler. It will help keep the some of the heat build up and stress of towing down.