Is you do a search on the M119, you will find that 300k miles will be a piece of cake. I know Steve Brotherton has a customer with something like 650,000 miles on one with nothing other than new rails and a timing chain or something along those lines.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record