How to replace throttle cable?

I've got a JCB 1550-B. Long story short, the throttle cable needs replaced. New cable arrived today. Seems the new one covers both the foot and hand throttle, which is nice.

Here's my question...

On the fuel pump side of things, the cable has a threaded end on it. My original has same threaded end...fine. The end fits into a spring and evidently, there is also a nut on the inside of this spring that is too small to come OUT of the spring. The spring is probably 3" long, 3/4" diamater and...(how to describe this) has this threaded nut inside of it, like a prisoner.

Perhaps I'm supposed to buy a new spring with this (my ignorance if so). I didn't do it.

how do you disassemble these? Over the years, the nut/threaded end have of course metamorphized into a single frozen unit. I've sprayed PB-blaster on there so it's soaking right now. Right now as I try to unthread the rod, the nut keeps turning/twisting inside the spring. I might add, the spring is pretty stout.

I'd like to simply unscrew the spring off, thread the new piece on BUT it seems there is going to be a trick involved.

Anyone know the trick?

Here's a picture of what I'm up against. It seems there is a nut inside this spring.