Yep, what was said.
The engine will fit obviously (same basic block design and engine family), and even the pan will "fit" into the cradle in many cases. But it won't be long for this world once you start driving around.
I've seen front sump pans in Broncos before, but never successfully. Can't say for sure that a '66 Mustang pan is any better or worse than the others I've seen. But I bet it's pretty close to not working. I think for the most part, Ford kept the same basic pans across multiple vehicle lines. So a Mustang pan might be the same as an LTD pan of the same era, etc.
But it's not right.
So the best thing to do is get a Bronco-specific pan and pickup tube and use it.
By "Bronco-specific" though, these days that includes the Fox Body Mustang, Ford Racing, and other "double sump" pans.
The Bronco pans were simply 6-quart "rear-sump" and the Fox Body pans have a front and a rear sump and drain plugs in each (and may be only 5-quart as well?). Very messy, but still the pan fits.
Even Explorer pans fit, but just barely. And the aluminum Explorer pans are not desirable because they can come in contact with the axle. And if it's hard enough, obviously adios pan!
Paul