Had a 1976 F250 with a 300-6. It had 180,000 when I bought it and I put on another 130,000. I did rebuild the motor at around 225,000 and found it had 30 over pistons already. I had to go to 60 over. It was still going strong when I traded it in on a brand new 1990 Mazda B2600I. I ended up with 251,000 on it, and still on the original clutch. It was running great also. My current rig, a 1995 F350 7.3 diesel, I plan to drive till until one of us wears out. Its at 198,000 right now.
