I've got a 351M in a 79 f150 that I picked up cheap which was supposedly rebuilt and set for a about a year without ever being fired up at all not even after it was rebuilt, i put some oil with break in oil additives and water in it and hooked up everything and started it and it took right off surprisingly, the engine ran fine, I ran it without the rad cap on to let any air out, but after about 5 minutes it began to boil out of the neck of the rad and the gage was at the mark before "H". Next I took the thermostat out to see if it was faulty but I had the same results just it took longer for it to overheat. I used a Multimeter with a Fahrenheit water temperature checker or what ever it's called, and sure enough the water was around 230°F. I also made the mistake of shutting the engine off with the cap off and it shot a geyser as high as the hood with hot water. Any thoughts on what this could be? Another point I had the heater on and it worked fine, thanks!