Transmission!!???!!! HELP!!!

Whatever that is is not A100 nor is it an overdrive. Chrysler did not make an OD 4 speed with a drive flange as shown. That is an A833 4 speed, and the photo is hard to deal with but it does look like an A body. You will need a 4 speed "floor hump" to get that to fit as well as a proper shifter. It will NOT hook to your column shifter.

What is wrong with the current 3 speed?

On a side note, what "we" used to call "an overdrive" was a 3 speed box with overdrive WHICH HAS an electrical solenoid sticking out the side. In the mid 70s Chrysler began modifying A833 4 speeds to make the 4th gear shifter position an overdrive gear. This is actually 3rd gear, but the shift lever was turned upside down, so as you shifted the knob 1-2-3-4, the transmission was actually shifting 1-2-4-3. Third gear was replaced with the OD gear. They do not exist in the older "flange drive" box like you show here