Not sure where to start. I'm think of pulling out an 6.7 and putting in a 8.3 cummins. In a kenworth T 300. How do you trust where motor is coming from? Wiring harness. I have a 2007 with 350,000 mile and a 8.3 I've never had a problem. I also have a 2008 6.7 with 125,000 miles and 10,000.00 in repair and a lot of learning how to keep working. I'm thinking is time to move in a different direction.