I was considering adding ghost ring sights to my Remington 870. Can the rear sight be mounted near the end the barrel where it meets the receiver or does it  need to be mounted rearward on the receiver, due to the small aperture size? Thank you for any thoughts on this. I'm just not sure I'm up for drilling and tapping the reciever.

The Wilson/ Trijicon type need to be mounted on the receiver as the base is contoured for the receiver. I installed the Trijicons about 1 month ago. It wasn't too bad. If you are unsure- send it to a good 'smith.

In order to work properly Ghost rings (Actually just a catchy new name for peep sights) MUST be mounted as close to the eye as possible.

The farther away from the eye they are, the slower they are to use, and the worse they work.
The whole idea of the aperture sight is to look THROUGH the rear peep, and allow your eye to automatically center the front sight and the target in the sight.

It's this automatic centering that makes the peep sight so fast to use, and so accurate.