Dies when you put it in gear

What engine does yours have ? 3.8 or 4.6 Do you have a professional type scanner that will show sensor data parameters ? Is the check engine light on ,or anyother light's ? Wrench symbol .
012 Idle Speed Problems Idle Speed Wrong I know yours only dies when put into gear , idles fine in park an neutral . Just showing what scan data is , if your not familiar with it. With the problem your having it's a necessity to have a scan tool that can view data pid's . You could spend a lot of buck's just replacing part's on recommendations from people on here . Testing an not guessing is the way to find the problem .

who puts a 4 cyl in a mustang??? 4 cyl cant push that kind of weight,i have a briggs and stratton motor that might work better

Start by checking for vacuum leaks, compression then a scan for fault codes. One of those should turn up a problem

I would check the timing could have jumped a tooth on the timing belt/chain. Also check the rotor in the distributer make sure that it is not worn out. I have also seen a plugged catletic converter cause big time power loss and really strange problems

Automatic or stick ?
If its an automatic, it could be something in the torque converter or transmission. The torque converter clutch could be stuck.

check fuel pressure -fuel filter/fuel pump--also check for vacuum leak or hose disconnected

it sounds like either the torque converter solenoid is stuck or the torque converter itself is bad

if the nuetral safety was a problem the car would not start at all, sounds like you have a defective engine wiring harness, this is the most common cause of this i have seen while working for Ford.

It's got all the symptoms of a leak in the fuel line. You may want to start backward at the tank and work your way all the way to the carb to find it but it would be surprising if it was something else. All the best.
Greg

he diff between a lockup vs non-lockup torque converter is the lockup simply locks the torque converter to the transmission direct electrically so there is no slippage in high gear. And there is no advantage to using a 2300 rpm stall converter if you are running a basically stock cam. However if you insist on this high stall speed torque converter you'll be able to lay down some impressive black marks froma standing start! One final note...unless you change your rear end gears your RPM's at a given speed remain the same. All the 2300 rpm does is your motor will rev a bit more before you start moving