Serkan, it is probably a 4.56 unless it has the optional 5.15. Your truck would have come from the factory with an Eaton HO52 rear end.
The tallest gears available for that rear end are 4.10. They were available on 1967-72 Chevy trucks and are somewhat scarce. The only way to find them is to search salvage yards or to watch 'for sale' listings. (One listed for sale now in the swap meet section.)
There is a Tech Tip describing this in detail.
http://www.stovebolt.com/techtips/axles/1%20Ton%20Upgrade/Matt
