I've learned a lot from old plumbers that were faced with this. They
drilled a series of holes ~ 1/8" in from the perimeter of a 3 1/4" cast
plug that was stuck solid. Cold chiseled the in between material and
knocked out that inner circle. From there it is a simple matter to
collapse the threaded portion to the inside with a chisel and it drops
right out. Easy on the threads too.
The post about heating the pipe is good but it will take a good torch
to do the job. Use at least MAPP gas or better Oxy MAPP.
Richard

Bill wrote:
You can fit a metal water pipe over the handle of a pipe wrench. In one
nasty situation, I used two large pipe wrenches and placed 6 ft. long 2"
metal pipes over the handles of each.

The 6 ft. long pipes on the wrenches gave me a lot of leverage.

Then I had one person hold one wrench in the counter clockwise direction
while I worked the wrench on the adjoining pipe in the opposite direction.
Even with all that leverage and pounding with a hammer, it still took a lot
of force to get that old water pipe unscrewed!