If its a six and not been changed, it will be a 226 with a side mount dist, if V8, 8ba or 8rt, dist, in front of the right side head. If orig V8, it will have a drain **** under each water pump. Have also been told that the first 8ba in the 48 trucks used head studs instead of bolts.
It doesn't have a bed. Was it probably a wood bed that rotted or fenders that are were removed? Did they have both options?
It looks to me like you have the factory short running boards (rounded off past the doors), so your truck probably did not originally have an express bed (the fendered, boxed bed). But it is a pretty old truck, so the bed may have been removed and the running boards swapped out some time in the distant past.