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What do you mean by a "valve job"? The 4.0 SOHC, or just about any motor these days and for the last 35 or so years, shouldn't need a "valve job" like the valve jobs of yore. The valves and seats are designed to work with unleaded gas, and the actual "burning" of the valves and loss of seal is unusual.
On the other hand, the 4.0 SOHC is a trouble prone engine in the cam chain department, and issues there can cause the cams to come out of time and may result in bent valves.
Serving suggestion based on prior posts here, one of which declared that 4.0 SOHC to be a "hateful engine" after a rebuild:
1) Pull your engine
2) Remove intake, anything electrical. Hold on to the rest.
3) Find a later model 4.0, even out of a Mustang.
4) Compare to yours, swap parts as needed. (exhaust manifolds, front drive maybe, stuff like that)
5) Replace the donor intake electrical etc with the original parts from your Exploder.
6) Install, drive.
Of course it's a little more involved. But a rebuild is complicated and expensive. You might get by with new heads and timing chains, guides and tensioners.
IMO replace guides and tensioners in any "new" motor you install.
I've heard that some mechanics refuse to work on these POS engines because getting the valve timing correct is a real PITA
Be sure the mechanic knows how to set it up properly.
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