Eric stokes and doubleA Have good points. I use extension rings on all my guns (just ask at the gun store to show them to you) Weaver, Talley, Millet, Burris, et all make them.
I have the same problem. I have to strain my head forward in order to take full advantage of the scope image. So I use both extension rings and scopes with long eye relief. Some of the best in this regard are Zeiss Conquest (4"), Sightron S2 (4.3") and Nikon Monarch (4"). But be warned. This measurement is from the glass to the optimum viewing distance. That little lip of tube that sticks out past the ocular lense is anywhere from 1/4" to 5/8" closer to your eye than the glass. And that is what'll get you!
There is something else to consider. Experts talk about scopes having "forgiving eye relief". This means that if your head and eye is a bit out of alignment of the scope, you can still get a good sight picture. I had a scope that was so unforgiving that you had to keep you head in PERFECT alignment or all you would see was black (this is called vignetting). I find the Zeiss Conquest and the Weaver Grand Slam to be very forgiving.
Also consider that if you get too close to a scope, you get vignetting too.
You may want to consider a low power scope, which won't be a hinderance in a slug gun
Let us know what you get
okcmco