After soaking in a penetrant, if the nipple will still not come out with the normal wrench, DO NOT twist off the lugs. Instead, proceed as follows.
Get a good quality, hard nipple wrench. If possible, make one out of drill rod and harden it. Remove, or don't put in, the cross piece.
Chuck the nipple wrench in the drill press, tightly. Pad the drill press vise with copper or leather jaws. Put the cylinder, rear end up, in the vise and tighten down, tight!
Adjust the vise and table so that the drill chuck with the wrench is directly over the nipple to be removed, and lock the vise and table.
Bring the nipple wrench down over the nipple, turning the chuck as necessary to line up the wrench with the nipple. If the press has a lock down, press the head down tight and lock it. If not, hold the press head down tightly.
Then grip the drill press chuck as tightly as possible and turn it. It may be necessary to work it back and forth to loosen a stubborn nipple. You can use a short rod the size of the chuck key for additional leverage, but it probably won't be necessary.
If a nipple will not come out with this treatment, it will likely have to be drilled out.