I was thinking mabe along thoes lines (flange spacer) although the cast iron flange has an inside diameter of 5". has anyone seen this size before? the 4"spacer will cover the 5" flange but there is some overlap on the inside since the 4" spacer is slightly smaller. Although with some silicone sealant this might work but I have my doubts.

There is no need to attempt installing a new closet flange. Begin by cleaning the top surface of the existing flange as much as you can. Using a stiff putty knife like a painters 5 in 1 paint tool will be sufficient. You are correct, the rubber gasket side of your buildup flange goes on the underside where the gasket will be in firm contact with the existing flange. I like to add a liberal coating of a clear silicone called "Plumbers Goop" on the rubber before setting it in place.(You can find a 4 or 5oz tube of plumbers goop at most hardware stores on the rack in the plumbing section.) The top surface of the flange should be about 1/4 higher than the finished floor. Place a standard wax ring on top of the buildup flange and set the bowl. The closet bolts are connected to the existing flange and extend through slots in the buildup flange and on through the base of your bowl. As the nuts on top of the bowl base are tightened it will hold everything in tight register and that should solve your problem.

Thank you for your considered and thoughtful responses! As far as explosive sewer gas, appreciate the warning but, as stated, already torched the vertical "tailpiece" and removed it. With respect to fernco fittings, I looked at these and found eiher a no-hub "sleeve" type connection or a "donut" which fits inside existing pipe. Doesn't seem the no-hub sleeve would provide the stability you would want and, at any rate, it does not fit over the coupling (hub) on the closet bend. The donut is designed for cast-iron pipe and will not fit in 3" coopper hub. Box-store people had no idea how to fix this as they are familiar only with cast-iron or PVC setups. I bought a new copper flange and was lucky enough to bump into a plumber at the supply house who gave me a few inches of 3" copper (10' of 3" is super expensive!). I resoldered this new "tailpiece" into the existing closet bend with my little propane torch (sorry again about torch and sewer gas, but too late). Hoping and praying that this joint holds up. Now I need to attach new copper flange - now you've got me worried about soldering it on. Hope anyone else who has to deal with refitting to copper closet bend can benefit from your comments.

The flange extender does not require screws. The original flange should be screwed to the floor, then the closet bolts are set in the old flange and the flange extender is laid on top. Once the bowl is set the closet bolts passing through the bowl and extender to the original flange will keep it all in proper register.