I went thru the same problem/. I also changed to alum pan and provided gasket. I torqued it properly. Do not over tighten. Next common leak is dipstick tube. Be sure you havre the correct tube for the tranny. It will take either an "O" ring or the top hat plastic type. Be sure your dipstick tube is supported and be sure it is in the correct position. This can be a tuff one to find as the kick down cable is near the dipstick hole and it can leak also.
The speedometer hookup is another culprit and on mine I finally figured out that the brand new driven gear was letting fluid past it. You can get a new bullet to with a new "O" ring for it.
And lastly on my tranny filling it to the full mark was to much fluid. I let it run a little lower and it is better.
All these places leaked on mine after a rebuild. I would go for a cruise and come back and crawl under and look to see where the leak was.
Plus of course I had to wipe everything off . , . Finally got them all.