The only difference between a 2 3/4" gun and a 3" Magnum gun is:
The barrel.
The ejector.
To shoot 3" shells, all you have to do is put a 3" chambered barrel on it, and have a gunsmith with the special riveting tools needed for the job, to install a longer 3" Magnum ejector assembly in the receiver.
I don't know about your local gunsmithing situation, so I can't recommend anyone.
The problem with installing a ghost ring sight set is the FRONT sight.
Remington brazes the bead's base on the more recent guns, and you have to cut the base off.
Once the base is off, you have to braze a new sight on.
This requires refinishing the barrel, and re-polishing the bore.
One option, is to send the gun to Wilson's.
They have a "Remington Steal Deal" where they completely refurbish your gun, convert it to 3", install synthetic stocks, and parkerize it.
I would assume they would also install their ghost ring sights at the same time.
Another option is to buy a new Remington barrel with the front ghost sight already installed.
Remington's Law Enforcement site lists a new 18" barrel with a factory ghost front sight at $143.00 list price.