Re: Threaded Muzzle break
Switching barrels is a pretty simple task with the right tools. As ewallace stated having the recoil lug pinned helps a ton for re-alignment. You'll need a good barrel vice and the proper action wrench (s). Removing a factory Remington barrel can be a bit of trouble. Remington uses a thread locker and the action is snugged up pretty damn tight too. It might be best to have the factory barrel removed by a professional smith first. I have a 20 ton press that I hold the barrel in between 2 oak blocks sandwiched between 2 steel plates. I use a clamp on style wrench to remove factory barrels plus I need to heat the action up with a propane torch first to melt to glue. Lock up the action wrench and then hopefully with a good hard blow on the action wrench handle with a 3 pound hammer break the action free. Once it's removed clean all the crap out of the threads and it will go back on fine with just a rear entry action wrench.
As for the muzzle brake question it's probably best to just install 1 brake per barrel and not try the switcheroo with brakes. There are just as many different thread sizes as there are brakes.
As for the stocks, you'd need to have the barrel channel cut so that it accepts the larger of the 2 barrel contours. Just remember you'll have a lot of clearance around the smaller barrel, it it will look funny but would work.
Switch barrel guns are a pretty economical way to make 1 gun into 2 with little expense. Most guys have switch barrels in 2 different calibers though. I had a switch barrel that was .243 Ackley for woodchcucks through out the spring and summer and then switch it to a 30-338 Win Mag in the fall. The were both the same barrel contour though. The only other thing I switched out is a different bolt with a magnum bolt face. They were both single shots on a Remington long action. I was able to keep the same stock and scope, just re-zero when I switched over.