One more tme. YOU'RE DOING IT THE HARD WAY! And not only that but you're doing it backwards. Didn't you read my post? And what size is the stack anyway? We have never used Duel-Tite gaskets on a cast iron vertical stack. We used them under the slab but above we used No-Hun Bands. Attempting to use donuts on anythng but a horzontaln installation is, in my opinion, a lesson in futilely. ThE reaon being that you have to lever the pipe into the donut, WHICH IS IN THE HUB and it's difficult to get a purchase for leverage when it's verical. Now you can be stubborn and insist on using a donut but if you want to do it the easy way you'll follow my instructions put forth in a earlier post. And by the way, Duel-Tite has a lubricant it recomends for their gaskets. I also see you have checked Fernco Neoprene Couplings. Bad idea for anything but a temperary repair and certainly not on a vertical stack.
Regards, Tom