I had a 75 F250 supercab that started life as a 390 camper special. The truck had 13,000 miles on it and was traded at the local Ford dealer. The previous owner had problems with the timing slipping . My dad bought and drove it to Fla and had problems there. I flew down to drive them home as dad had gotten sick there. I adjusted the timing and off we went. Got back home and did a valve job and changed the intake. Truck kept showing the same symptoms. It ran but dad didn't get to drive it all that long as he passed away. I got the truck and promptly yanked the 390 and installed a 460 in it using all factory parts. I ran that truck for 10 yrs until it rotted away. It got 14 mph loaded or empty would cruse at 70 all day long. I finally did find the problem with the 390, it seamed the timing chain was defective right from the factory. Took a boat mechanic to find it. long story short, you could put the 460 in the earlier chassis and make it look stock.
