Howdy,
Here's yet another vote for the rad. On a '75 C-10 I used to own, I went through all the same stuff with the 250. I had the rad done and still no success. The only thing that fixed it was a new one, and it was a stock 2 row for the 6 cyl! It didn't look like much, but it worked like a champ!
best wishes,
Les
