Before you go planning a V8 swap, just diagnose the problem. Hopefully, you didn't crack a head or ??? when you overheated it. (Yes the V8 will cost a lot and so will a "new" 4.0 - but we are getting ahead of ourselves.) There is a "freeze" plug in the back of each of the cylinder heads - they could leak. The heater hoses route over the motor, one of them could have leaked, there is a way water can leak from under the alum. intake at the heads, yada yada. You have a nice truck, diagnose the problem and then fix it.