I'm trying to choose my gearing accordingly.

Here's my thoughts, thus far:
215/50/15 - 23.46" diameter - all around street tires for "everyday" driving
225/45/15 - 22.97" diameter - autox / timetrial tires

now, with my street tires (215/50/15) on my modified transmission, this will put me around 4650 @ 80mph which is a little high IMO.

What I have considered is finding some wheels with a low offset (like +50 or something) that are narrow so I can stuff a tall narrow tire in my wheel wheel for long-distance driving. A 26.5" tire would bring me down to a very near stock GSR 5th gearing. Would this even be possible?

Appearance and performance do not concern me with these tires.

 

26.5" is doable IF you are at almost stock ride height. My buddy cleared 235/60/15 BFG Drag Radial 2's (26.1"), but he had to raise his coil overs to near stock ride height.