904 vs 999

It's odd that you would have a bang as you describe it when going into reverse since you have a 1982 transmission. Chrysler made a change in 1978 to soften the engagement into reverse.
The following is a description of the change in reverse line pressure.

All automatic transmissions starting in 1978 are equipped with modulated reverse line pressure that provides controlled pressure in reverse gear.

Previously reverse line pressure was a constant 260 psi throughout the full range of throttle operation. This pressure is now modulated from 150 psi at idle speeds to 260 psi at full throttle operation.

This change reduces the pressure to which the front clutch seals and reaction shaft seals are subjected,providing a beneficial effect on transmission durability and reliability.

This improvement is accomplished by routing the reverse control fluid to the regulator valve, reducing the fluid pressure at closed throttle to 150 psi.
At wide open throttle , the regulator valve provides the 260 psi needed during heavy duty reverse operation such as backing up grades or parking a heavy trailer.
However, during normal reverse operation such as backing out of a parking space or driveway, the system is only subjected to a pressure of about 160 psi.

There are various ports on the transmission case where pressures can be checked with a simple 0-300 psi gauge.
Those numbers can be found in the service manual.
A good diagnostic tool to solve many problems,