Another idea. I put new rings and a piston in my '57 6400 grain truck with a 261. It ran fine but would start cutting out and die after it went about 2 miles and got warm. Drove me nuts. I searched on the web for this type problem and at a tractor restoration site, it said this problem could be caused by a faulty condenser. I put new points and a condenser in and it has not done it since. Unusual, but cheap and simple to try.