How much peril am I in trying to remove a nut on the base of the trap to replace the chain? I can see where there's a definite danger of marring the nut and maybe worse. Anyone ever done something like this and if so, how did you go about it?

compress the spring where you are adding the chain.
leave the spring clamp on the spring or replace with a u-bolt, your choice. remove the nut, if you are afraid of marking the nut wrap jaws
with tape. remove nut place chain on the spring an reverse the operation.
Lubrication is your friend.
pick65

I would be more worried about snapping the post off than damaging the nut.
Breaking the jaw post is something I would do.

I imagine the nut has been on there the better part of one hundred years and I'm not looking forward to removing it. The original chain was replaced at one time or someone decided to beat on it with a hammer and elongate the ring on the end of the chain. Can't tell for sure.

I'll spray it with Deep Creep and let it set a few days. I'll let you know how I make out. Thanks for the input.

Spray nut liberally with a penetrating lubricant at least 2 days before loosening (submerging in WD-40 is best).
Place trap upside-down in vice with jaw post nuts facing up and spring compressed.
Tap (not beat) evenly on center point of all sides of nut with small hammer...stay off edges of nut. A square cobbler or upholstering hammer works great.
Repeat 2-4 times. Be careful not to scar up base.
Nudge (don't force) nut back and forth (slightly tighten first, then loosen) with large adjustable wrench until it breaks free...spray threads with lube ahead & behind nut during this process. On very old traps this step is where you must have patience and work it back & forth slowly.
Clean & lube jaw post threads before replacing nut.
This is how Gov't trappers removed nuts to change thousands of replaceable parts on NH traps.

Thanks IVM...excellent insight....I admittedly have tried to muscle some of these nuts loose before and like Ol' Smoke mentioned broke jaw posts...an expensive lesson.

Ditto. The chain on the trap may very well be the original chain beaten out of shape. I bought a vintage chain that is a match for the #15 just to bring it back to its former glory. I'm going to give it a try.

I would take and soak the nut with Kroil or PB Blaster for a few days and randomly hit it with a hair dryer. You want to get it hot but not real hot, maybe just where it's uncomfortable to hold. Between the oil and the heat it should break loose, the heat helps the oil get in the threads better. I've done this with rifles when I want to rebarrel them and it sure helps, some come loose amazingly easy. I can't see why this wouldn't work on the trap nut.

Me being a carpenter, I would use a small hardwood block
between the hammer and nut...to tap on.

Thanks for the help, guys. Hopefully I'll have a good report for you. Chain hasn't arrived yet.

I heat with propane torch,trap upside down, put oil on threads,when you see oil bubbling on threads ,its penetrating them.Don't be in a hurry,heat nut only this will expand metal letting oil work in.slowly turn nut back and forth, alot of rust going to be in there.once it breaks free keep turning back and forth, slow is better.keep putting oil on for lube.

If all that is wrong with the chain is the ring is mishapen I would have left the trap all original not changed nothing and just paid a black smith to reshape the ring, cheaper than buying a chain and changing it out imo.