mating a vent with ABS

I'm working on a bathroom remodel. The house is 40+ years old on slab with cast iron DWV.

I need to cut the vertical vent that goes out the roof (single story house) to add a ABS T below grade (I'm trying to mate a new shower up with the existing DWV).

So, I'm trying to figure out the proper way to do this. I'm thinking about using Fernco type rubber fittings to mate between the existing DWV and the new ABS.

But my real question is this OK given that I've got a pretty heavy vertical metal stack above (I think it is 1 1/2 inch) that goes up through the roof (1 story). Is there an issue with having this vent sitting ontop of the new ABS fittings I'm putting in below (probably mated with the Fernco?).

If not, what's the proper way to do this? Pull out the vent and replace the whole thing through the roof with DWV?

Thanks,
Steve

As you realize, cast iron is damn heavy. I removed my CI vent stack once and even though I thought I had it well secured before I started cutting, once the support of the lower end was removed, I had a 15 foot length of 4" CI pipe crashing on to the basement floor! I'd replace the stack with ABS or PVC because of the difficulty of supporting the weight of CI as well as ease of working with ABS/PVC, but just exercise extreme caution. I'd consider starting at the top and working down taking relatively small bites at a time if this is possible. Have help (muscle).

Mating Steel/Cast Iron to ABS DWV

Actually, I think I solved my own problem with some more research, so I'll post for those in the future.

The way you mate an existing cast iron or Steel vent to new work ABS is to
1) support the vertical metal vent with a pipe strap that is attached between the wall studs.
2) Then mate the iron/steel to the Plastic DWV. This takes the potentially heavy load of the metal column off of the plastic pipe (and whatever is below).

Apparently, since cast iron is quieter some plumbers/remodelers like in some cases to use cast in a wall for section that may be near a room that they want to keep the noise down. In this case, metal pipe is "spliced" in between PVC/ABS. The Metal sections are then clamped and strapped.

They make some nice pipe clamps that clamp the pipe and then can be either supported between studs with added blocking or with a hole drilled through the stud on each side. UNFORTUNATELY you'll never find these at your local Lowes or Home Depot. I've only been able to locate these on-line as even my local real HW stores had no clue what I was asking for.

In my case, since I was trying to get a job done, I ended up fabricating something out of EMT conduit straps. Not great but did the trick.