Yes, it looks to me like the rear proportioning valve in that picture. More rear brakes with more weight.
On the engine bay, there’s something there I don’t often see. Those two air pumps. If you don’t use them please don’t toss them. Something someone trying to register a truck that needs all the emissions stuff on it would like to have.
I don’t remember, but do you need to pull the heads? Might it run ok without tearing into it?
Good engines, low end torque and drivability are natural to it. MPG, no!