Okay, this will be a picture-heavy thread, consider yourself warned. I have found that there is a severe lack of pictures on this, so I dove in with camera in hand. If you see something that is incorrect or incomplete, please chime in, but only if you can explain the CORRECT way in a comprehensive manor - pictures prefered.
Remember there are many different ways to modify an automatic transmission to get the same effect, I am only attempting to document one of those ways.
Here goes....
Pull the cover. Remove the filter, the valve body is under it.
Remove the valve body. If you are very carefull, you may be able to reuse the gaskets if they are undamaged. There are 2 gaskets with a thin plate in between them, we will be drilling 3 holes larger in this plate. The gaskets are labeled with a "C" and a "VB" to tell them apart. The "C" goes next to the case, and the "VB" goes next to the valve body. The detent solenoid will have to be unbolted as well to remove the gasket.
Now you can see the check balls, there will be up to 6 of them. Don't panic if yours has less, some had less, and someone may have been in there before you.
Note where the 3 arrows point. These holes will be opened up to a maximum of 3/16", any larger serves no purpose as this is the limit of flow anyways. (The hole size can be smaller, but I'm after a firm shift so I went to the maximum.)
These 3 balls can be removed, as their purpose has been deleted by the drilling of the plate. You may have more or less balls, but make sure to leave the frontmost ball & the rearmost ball.
This piston is the 2-3 accumulator valve, its spring can be removed to firm up the 2-3 shift even further.
Replace the piston without the spring.
This mod is to allow us to hold 1st gear manually without the factory upshift at a specified level. Remove the pin with a pair of needle-nose pliers.
With a magnet, remove the piston. Keep things in order & CLEAN.
See the black mark on the ringland on the right? Grind a "flat" area on this to allow holding 1st gear manually.
"Flat" ground.
Reassemble everything & try it out. Things should feel a little different than the factory setting, although even the factory used these settings on select performance models, so don't worry that you have done something radical to your transmission, these modifications will actually help the transmission last longer, as the soft slow shifting (and subsequent wearing of the bands) is now greatly reduced.
Enjoy!
Remember there are many different ways to modify an automatic transmission to get the same effect, I am only attempting to document one of those ways.
Here goes....
Pull the cover. Remove the filter, the valve body is under it.
Remove the valve body. If you are very carefull, you may be able to reuse the gaskets if they are undamaged. There are 2 gaskets with a thin plate in between them, we will be drilling 3 holes larger in this plate. The gaskets are labeled with a "C" and a "VB" to tell them apart. The "C" goes next to the case, and the "VB" goes next to the valve body. The detent solenoid will have to be unbolted as well to remove the gasket.
Now you can see the check balls, there will be up to 6 of them. Don't panic if yours has less, some had less, and someone may have been in there before you.
Note where the 3 arrows point. These holes will be opened up to a maximum of 3/16", any larger serves no purpose as this is the limit of flow anyways. (The hole size can be smaller, but I'm after a firm shift so I went to the maximum.)
These 3 balls can be removed, as their purpose has been deleted by the drilling of the plate. You may have more or less balls, but make sure to leave the frontmost ball & the rearmost ball.
This piston is the 2-3 accumulator valve, its spring can be removed to firm up the 2-3 shift even further.

Replace the piston without the spring.
This mod is to allow us to hold 1st gear manually without the factory upshift at a specified level. Remove the pin with a pair of needle-nose pliers.
With a magnet, remove the piston. Keep things in order & CLEAN.
See the black mark on the ringland on the right? Grind a "flat" area on this to allow holding 1st gear manually.
"Flat" ground.
Reassemble everything & try it out. Things should feel a little different than the factory setting, although even the factory used these settings on select performance models, so don't worry that you have done something radical to your transmission, these modifications will actually help the transmission last longer, as the soft slow shifting (and subsequent wearing of the bands) is now greatly reduced.
Enjoy!