What's the feasibility of having my #1 in .416 Rigby made into a takedown? I'm going to pull the barrel so I can have the action case hardened, so it would be easy to have tit converted at that time.

I came across this picture a number of years ago, I thought it was interesting. Not a take down like some of the lever guns we see but interchangeable barrels without the removing the forestock.

There is also a youtube video showing a set. It seems possible but I already have so many other take downs that it does not make much sense for me to make another one this way.

Problem is that you can't have a rib, or scope mounts on the barrel; every time you want to change barrels you have to remove them. IF you use a threaded barrel.
I can't tell about the one in the picture but I don't see threads but I would like to know how they did it.

dpcd,

would you think it feasible to mill the threads at 180-degrees like a Savage 99 and add a set screw like the Stevens 44 1/2's on the bottom?

yes. but the set screw can't be on the bottom as the hammer and mainspring are there. Maybe on the side.

dpcd,

is that a vague hint that I should start shopping for a #1...?

A set up in 300/375 H&H would give you something to do this upcoming winter...

Ok, he uses the interrupted thread system so you can have a scope permanently mounted. But he is also the one that butchered up the double rifle a couple of years ago.
Something to do this winter? You are evil.