I've found a 1980 F-250 with a "recently rebuilt" engine. I dont know what it is but the thermostat housing is on the block and not on the intake manifold. Looks like a 351M 2 bbl. No tags are left on the valve covers or under the hood at all. I'm not that sharp on ford engines so I'll turn to you for help. The vin is F25GKGG8959 as taken off the driver door. Thanks, neals

 

It probably is a 351m-400 if the thermostat is in the block. Ford was still putting them in trucks in 1980.