Chuck.
Don’t know about the Seneca but on my New Englander I had to drill and tap one hole. Easy to do. IMO you want the peep as close to your eye as you can get it. However, you must also be concerned about the rear angle so as it does not sit low. If you simply set the peep up on the tang, you will see which existing hole to use. If you have a wedge it might give you the option of using either existing hole.
The other consideration is how close to the hammer the sight will be. Personally, I do not like the peep close to the hammer.
This one is a conical shooter so it required the peep to be almost maxed out on elevation adjustment when using the existing rear tang hole. Depending on the angle of the Seneca, and if you are going to shoot round balls, you might get by using the rear existing hole, in terms of elevation adjustment.
Good luck.
