Wow...he says the truck was a 360 originally...people do swap the original engines out over the years on these vehicles, "factory installed" doesn't mean squat anymore in many cases.

Here's an FE engine (360 or 390, both look exactly the same.)
Identifying features:

Intake manifold runners partially covered by rocker covers, as John said. The exhaust port runners extend out from the head ending with flanges that have the top exhaust manifold bolt holes through these flanges.

5 valve cover bolts, very rectangular shape to valve cover:



Vertical (top to bottom) exhaust manifold bolts (for most FE engines)

Distributor goes through the intake manifold This is the only Ford engine that has this feature.





289/302 features:

6 valve cover bolts Note distinctive shape of the 289/302 valve cover, where it angles in at the top:


Horizontal (front to rear) exhaust manifold bolts

Intake manifold curves around the distributor




The 289 is a very compact engine, so it will look virtually lost in the large engine compartment of these trucks where the larger 360 fills it pretty well. And if someone did change the engine and you're looking to change it back, they'd have changed the tranny too since the bolt patterns between the two engine families discussed here are totally incompatible.