[mcaldwell]

Do you just mean size?

5.8L which is 351ci. There was no 6.6L engine available in any Ford Bronco.

 

[mcaldwell]

Ahh, you may be right.

But there was no 400M available in 81 as far as I can recall. I could be wrong though, it happened once before.

 

[mcaldwell]

As I said earlier (and according to this site if you don't believe me), the 351M was the only option in a Bronco in 80, or 81. You cannot have the 400M unless someone swapped it in with a little work.

http://home.earthlink.net/~bubbaf250/history/history10.html

If you think it is not original, and want to go further, you will have to get a part number off the engine.