I'm not even going to venture a guess, on whether or not the previous owner put in a lower temp thermostat..
It is not out of the realm of possibility though..
The reason I say that is, because the 5.0 MN12 did not have a sufficient cooling system from the factory..
So in order to help
that problem, the previous owner could have installed a lower temp thermostat..
There's no way of knowing, without actually physically pulling it out and testing it though..
I've been a 5.0 MN12 owner for awhile now, so I know all of its little quirks..
To tell you the truth though, from a reliability standpoint..
The only issues the 5.0 MN12 left the factory with, was the cooling system..
You could also argue that the AOD falls into that category, but it's not going to leave you stranded anywhere if it fails..
Most of us longtime 5.0 MN12 owners have opted to swap out the stock clutch fan, for an electric Mark VIII or 94-97 MN12 electric fan..
Either one will work, the Mark VIII fan requires a bigger alternator to run it though..
You're not going to know until the hottest part of the summer, if you can get by with the stock clutch fan set-up..
I know you said you wanted to keep it stock, sooooo...
Since you want to keep it all stock, If I was you I wouldn't mess with the cooling system until it warms up outside..
You could give the cooling system a flush, if it looks muddy or rusty looking..
Then when it warms up outside, you can go through and change the thermostat and water pump..
As well as checking your clutch fan for functionality..
I would replace the current thermostat with a 180 degree Motorcraft unit..
If you're keeping the clutch fan set-up, then just replace the water pump with a factory Motorcraft replacement..
On my set-up, I chose to use the Mark VIII fan..I also opted to use a 94-95 Mustang (5.0) water pump..
The Mustang water pump does not have a threaded snout for a clutch fan provision..
Since the 94-95 (5.0) Mustang used electric cooling fans instead..
Don't forget to replace ALL
4 of the rubber hoses used in the cooling system as well..
These degrade over time, and their life expectancy is probably about up..
Just because they look fine on the outside, doesn't mean they aren't deteriorated on the inside..
In the summer, the needle on my cooling gauge stays on the "N" in normal..
Unless I get on it real good, then it will creep up to the middle then back down to the "N" in normal..
That's with a 180 degree thermostat, a Mark VIII fan, and a High Amp Alternator..
I'm running a stock replacement radiator..Which is sufficient with my current set-up..