Lots of confusion over the years when an engine comes up as being from a "Torino" or a "Falcon" or whatever. Just like was said, it was designed by that team, but used in multiple platforms and for multiple years sometimes. The sometime-exception to that is the "T" numbers which were in fact designed for the Truck line of Ford products. Might still have made it to passenger cars, but does not seem to be as often as the other way 'round.
There are often one-year only models, but more often than not that same number (with variations on the suffix letters of course) were used for two, three, or even more years in a row.
But the bottom line is it's still a '79 revision in a '76 vehicle, so that tells me that no matter what it's from or when it was built, it was installed after your Bronco was out in the real world and for some reason one of the PO's wanted or needed to change it out.
Might even be as simple as a shop rebuild, using an easy to find block, so that the vehicle downtime while the engine was at the shop was minimized.
Lots of scenarios. Do you know or have contact info for any of the previous owners that might shed some light?
paul