Short update...
So, thanks Bob, Kens videos are great, and to work on transmission with them really fun! My front band had to be replaced / relined so I decided to buy a flexible band. Unfortunately, use of this type of band requires other strut (as for 1972-73 fmx), so I’m still not able to assembly the trany – waiting parts to come. Maybe it might help someone, so I’m posting a picture how I solved installation of direct drum piston. I used thin plastic flowerpot (cut flash at bottom ad left some rest above for pulling it out) to press the seal ring, inserted inside the drum and by constantly pressing on piston I slowly removed the pot strip.
Till struts arrive I started to rebuild side louvres an rewire the car.