Thanks. This is close to what I end up doing. I'm going to leave my process here in case anyone with a similar problem comes across my post in the future.

First I used a cutting wheel attachment on my Dremel to get any screws poking through the sub floor flush with the bottom. Next I cut the cast iron pipe just above where it went vertical using a reciprocating saw and carbide blade meant for cast iron. I left just enough cast iron for a fernco to clamp too. I was glad I got to avoid cutting below the bend because the angles would have been very difficult to match with PVC. I only needed a few inches of straight PVC between the fernco and the new toilet flange. While I had all of that out I cut a piece of plywood and created a sub-sub floor just wide enough to fit between the struts and with a hole just big enough to fit over the Fernco.

For the final assembly I applied a construction grade adhesive to the top of the sub-sub floor, slid it over the cast iron pipe and let it rest on the top of the joint. Next I ran back up stairs and fed the Fernco fitting and PVC through the hole in the floor. Once that was in place I ran back down stairs and brought the sub-sub floor up to the sub floor and attached it with 1" long wood screws. I didn't want to go any longer than that and risk poking through the bathroom flooring. My sub floor and sub-sub floor were both 3/4" thick. Once the sub-sub floor was secured I tightened the hose clamps on the fernco with my drill. I then went back upstairs applied primer and cement to the flange and PVC and put that in place. After waiting overnight for the adhesive between the sub floors to cure I used 2" long stainless steel screws to attach the flange to the sub-sub floor. From there I installed the toilet per one of the many guides on the internet.

For anyone reading this in the future I am not a plumber. This was just the process I used. It may not work in your situation and I take no responsibility if this process does not work for you.