The cast iron pipe in the first photo is NOT a vent and cannot be used for one regardless of how close it is to the trap. ANY vent, atmospheric or admittance valve, MUST be connected after the trap and BEFORE the drain line turns downward, so it cannot be "a studor vent pointing straight up and as high as you can get it (dont jam against the floor, you may someday need to unscrew it to replace it)" because that would place it below the trap.
YES HJ, it can be a studor vent pointing straight up as high as he can get it. It would HAVE TO BE be above the trap it would have no choice.. Read the earlier posts, youll see exactly what I was telling tev9999.
Here it is so you dont have to look back.
Add a 3 inch tee directly above your adaptor and use the side branch for a new 3 inch cleanout. Bushing the tee down to 2 inch at the top of the tee. go vertical as high as you need to with 2" pipe. Add 90 degree 1/4 bend to turn horizontal.
Run horizontal branch to shower trap at 1/4 inch per foot slope. Somewhere in this line, within 8 feet MAX of the shower trap you need to add a tee (or wye and 1/8 bend) depending on local code and add a studor vent pointing straight up and as high as you can get it (dont jam against the floor, you may someday need to unscrew it to replace it).
Clearly, a shower drain is in the floor, placing the trap below the floor at some unknown distance.. adding a studor vent to the trap arm and standing it straight up
in the trap arm would have to make the studor vent higher than the trap.
Dont get me wrong, I'm not being argumentative, I gave tev9999 a plan for tying into the cleanout that was as correct and legal as it could be. He then asked about other ways to tie in this drain to the stack that is there. There are several posts about that plan and as you can see it got to be fairly confusing. Once it was clear that tev9999 needed to go back to the original plan of tying into the cleanout per the method I had specified in an earlier post I expected him to follow those instructions that I posted before and re posted up above, which are correct as to studor vent placement. Agreed?