With the Weight you have mentioned......there's No way your 4.3 Liter V-6 will be able to Tow 8,000Libs....
Below is from the 2005 Silverado Owners Manaul,
A 2005" Silverado Ext.Cab with V-6 with Auto Trans. and 3.73 Gears.....has Max Tow Rating's......is 5,200lbs...
Edit:....To Tow the Weight(8,000lbs)....a 3/4 Ton Truck would be the way to go.....for Towing that Weight Safely.