Capping off a propane line: need tips and tricks!

After great effort, I just now found my very slow propane leak (and I will soon prepare a fascinating ;) report on the many things I learned while trying to find it). Bottom line – the leak is at the point that the external barbecue fixture connects to the propane system.

What I want to do is remove the barbecue fixture – we never use it. And I want to cap off the stub or nipple that "tees into" the propane line. The pipe is galvanized. It appears to have an ordinary pipe thread -- not a flared brass gas fitting.

I am planning to unscrew the existing reducing bushing (I think that is what it is called) and replace it with a galvanized cap. I will use teflon gas tape at that joint.

My real question is this: I don't want to put any stress on the other galvanized propane lines underneath the trailer. I will hold back on the galvanized nipple with a pipe wrench while unscrewing the existing reducing bushing. The bushing is very rusty -- I anticipate having to use a lot of force to unscrew it. I'm planning to spray it with WD-40 a day or so before unscrewing it, to try to loosen the threads.

Does this sound like the right approach? I am not a plumbing or propane expert, as you can tell.

Thanks in advance for your suggestions!!