First gear is going to take you to just over 60mph with those tyres at 4500rpm and with the weight and gearing of the truck currently im not sure how much faster than that it will go over the quarter?
If you cant find a really cheap diff centre for it i would be putting the smallest 15 inch tyre you could find on it, even a 25.6" tyre like a 225 50 15 would only take you back to around the equivalent of a 3.5:1 ratio.
