Need Misfit Guidance on a 44-40 Rear Sight...

Should already be drilled an tapped for a tang sight, Lyman or Marbles. . . that one you posted is likely to be too far from your eye to be of any real use.
your mileage may vary

Picked it up off of a Cowboy action shooter, already installed, I found it nice for quick short shots, havent tried much further than 25 yds yet, the small ring may be too large for any accuracy work? Seems more like a ghost ring than an aperture.

ok. Without a real aperture sight I like to use the 2nd notch or shelf or whatever on a buckhorn. Easier for me to line up than the fine bottom notch. Try the bullseye and you can always switch back. Also, a 3/32 bead (even fiber optic if you like) may help. Never know until you try...