I have an f150 that is a 1978 but some parts are 1979. why?

Some car manufacturers use parts from past year models, it is still done today. Though you may have a truck that was repaired in the past using 79 parts. Im not sure on your engine question. Myabe some Ford truck fellas can ring in on the answer.

I believe the 351 was an option.  Krez is exactly right, the use of leftover parts is common practice among auto manufacturers in similar model years.  

also wndzman, if ur 1978 has a late in the year mfg month (see door panel) that could b the reason for the next year model parts. i can find no evidence of the the 351 modified being put in the 1978 from the factory, but i do think there were a limited number of 351 detroits in 1978, but i could b mistaken on that

I have a factory original 78 with a 351M.

Ford used to do this a lot. Thats why you don't have to worry about matching numbers on Fords. As long as you have the right engine size with that year model number you are good. I had a 69 Fairlaine with a C4. The casting number on the trans case was C60E, which is 1966 and the car had never been touched before I worked on it.