I've removed the cast iron and am dry fitting the replacement parts. I used a 4" - 3" reducing coupling, 3" pvc and a 3" 90 to line up where the flange needs to go.
Problem: With the 90 in place, the flange is going to sit about 3/4" proud of the subfloor. (The final floor will be a fiberglass material that compresses so I'm not really counting it)
Can I create a reinforcing support for the flange to sit on to get solid support, and if it sits that proud, will it affect the toilet? Or do I need to figure some way to lower the flange? The latter would be very difficult given the very tight space I'm in.
Update 5 Nov 2012
I found a 3" flange with a 45 degree bend and coupled it to another 45 bend to get what will be a perfect fit once the 1/4" ply (underlayment) and the fiberglass sheet flooring are installed.
The flange is actually above the subfloor by 1/4 inch, sitting on waterproof shims on all sides.