I have run into this a lot on older vehicles. Unfortunately, what happens is the mating surface on the transmission case itself becomes distorted or has some imperfection in it and so a new rubber gasket and pan cannot take up the difference so it leaks. If you have used a razor blade and cleaned the case side of the pan mating surface so that the pan and gasket has a good surface, then I would just use transmission RTV. This will take up any difference and seal the pan. Just make sure to apply this to the pan and that you surround each bolt hole to the inside edge of the pan with sealant.
Here is a video just in case you have not used this product before. This is a video of a rear differential cover so just make sure you are using sealant for a transmission.
Saturday, May 18th, 2019 AT 12:53 PM