I seem to have found a cheap/easy solution at Lowes. The ID of the
iron pipe is about 3 3/4" Picked up a Fernco 3" to 3" donut, a section
of 3" PVC and a 1 1/2" to 3" PVC wye. Cut about an 8" section of 3"
pipe and put the donut on so about 2" of pipe is below the donut. This
fits VERY tightly into the pipe - maybe too tight. Put the wye above
the donut - the wider bell keeps the donut from sliding up. Managed to
push about 1/4 inch into the iron pipe and gave it a full garden hose
of water - No leaking through the donut (didn't seal the PVC yet).
Disassembled for the moment since I'm not ready to run the rest of the
pipe yet. I'll wire brush the ID of the iron to clean it up and help
push the donut down. Cleanout cap will go in the top of the wye, sink
drain into the side.

The cleanouts are higher than the floor drains, so my sewage problems
back up on the basement floor anyway. I will pick up caps on my next
trip to Lowes. Learned two things several years ago:

1. Never try to snake a sewer line under a maple tree with a 50' flat
blade snake - it can get entwined in tree roots 40' out.
2. You never want to hear your plumber say "backhoe".