Take it to a mechanic and say "will you fix the heater core?"
If your asking if it can be done without removing the transmission? NO.
No, the 2003 manual transmission will not fit a 1991 Ranger.
Motorcraft MERCON automatic transmission fluid is used in the 5 speed manual transmission according to the 1997 Ford Ranger owners manual
The manual transmission on a Ford Ranger should be changed on a regular basis. Fill the transmission with any good quality regular or synthetic transmission fluid.
Motorcraft MERCON automatic transmission fluid in the 5 speed manual transmission of a 2000 Ford Ranger in North America
Motorcraft MERCON ATF ( automatic transmission fluid ) is used in the 5 speed manual transmission of a 1999 Ford Ranger , according to the owners manual
According to Haynes Repair Manual #36071, 1993-2005 Ford Ranger pickups, all models: Manual transmission fluid type is MERCON automatic transmission fluid.
Manual transmission fluid is MERCON automatic transmission fluid.
The manual transmission doesn't have a dipstick , there will be a fill plug on the transmission
Manual transmission fluid type ....... MERCON automatic transmission fluid. - Haynes Manual #36071 - 1993-2005 Ford Ranger Pick-ups.
A better question is why would you want to put a ranger transmission on a mustang
Manual transmission lubricant type: MERCON autmatic transmission fluid - Haynes Manual #36071, 1993-2005 Ford Ranger Pick-ups.