Replacement window thinner than space betw stops

Hi there, first time installer.
'm putting in vinyl replacement windows and I have 2 (for now) install questions:

(1) the space between my inside stop and blind (outside) stop is about 1/8" to 1/4" deeper than the replacement unit.
What is the best way to make up this space-
(a) install replacement up against blind stop, leaving 1/8" to 1/4" gap on the inside and remount inside stops against the the unit, closing the gap. The stool (horizontal ledge piece at bottom of window) would still not be flush against the replacement unit however, so there'd be a gap on the bottom. i could stuff in backer rod and caulk it i guess.
(b) put 1/4" thick trim piece between unit and blind stop, essentially shimming the window to fill in the space so there is no gap on the inside.
(c) something else?

(2) the sides of the replacement units are channel shaped, i.e. the front (inside) and back (outside) faces of the units are about 1/8" wider than the body of the units. When I shim, should the shim be against the body of the unit or the edges of the faces? what should i fill the "channel" with?



all suggestions appreicated. thanks!

I would line the window up with the stool so that your interior stops will line up exactly where they used to be. You don't want to have your interior stops be farther out, because it will leave an unsightly line there. Any strip you add to the blind stop will likely be covered in the end with a bead of caulking, so it doesn't have to be anything fancy. I would suggest that after you add the strip, you run a light bead of caulk on the inside corner of it (where it meets the jamb) to seal up any air that might happen to sneak past there from behind.

As for shimming, you will use shims that are the same width as the window (3 1/4?) and they will ride on the edges of the window, not the portion between the window. The "channel" of the window can be lightly stuffed with strips of fiberglass insulation, or "low expanding" door and window foam. I like DAP latex door & window foam because of the small nozzle and it has hardly any expansion pressure, so it's not possible to bow a window using it.

Thanks Xsleeper, that's very helpful!
yes the units are 3 1/4" deep, is that a standard size? i guess it wasn't back in 1922. seems to be 3 3/8" to 3 1/2" between the stops. i wish they sold 1/2" X 1/4" trim to fill the gap.

the "unsightly" line you're referring to- I guess you mean the 1/8" to 1/4" gap on the bottom between the stool and the window if the window is pushed up against the blind stop w/o any spacer. i might see how it looks, might be able to live with it if i can't find trim to space it on the back.