For $500 I would do it.
You will have a wiring nightmare!
The SOHC uses a completely different setup, especially from 98 to 91.
Now the ONLY way to do this conversion without a huge headache is to buy the entire donor vehicle, which you are planning to get.
Look for issue's with the exhaust hooking up, underhood layout is going to be a huge issue, relocate the airbox, battery, coolant overflow, etc....
The engine, computer, and power distribution wiring will be a nightmare if you dont know anything about stuff like this.
All in all you could probably spend the same amount of $$$ when all is said and done and get a 5.0l
Also like mentioned above the OHV 4.0l can be built into a monster that will toast the SOHC, I know I have one.
The SOHC can likewise be built into a monster but there are no aftermarket parts for it yet, so its a do it yourself deal.
It would be neat to have, a HUGE job to do, and the end does not justify the means, if you ask me.
I have done several 2.9L to 4.0L conversions and touched on a couple 5.0L conversions. Mostly I handle the wiring stuff, but we have passed over the SOHC swap simply because the 5.0L is a proven option for same or less $$$$$.