I have existing sliding aluminum-frame windows in my brick house. There's a brick sill, with the bricks oriented slightly downward and perpendicular to the wall. Kind of like this (except with aluminum framed windows):
On the interior the windows are framed with casing like you find around a doorway, so the interior opening is about 3.5-4.5 inches smaller than the brick exterior opening. Looks like this (except aluminum-framed):
I'd like to retain the existing interior sills. What kind of replacement window frames do I get? Do I get block frames such that the interior frame is just smaller than the sill, and fill in the exterior frame with wood? Or do I get Z-bar frames?
On the interior the windows are framed with casing like you find around a doorway, so the interior opening is about 3.5-4.5 inches smaller than the brick exterior opening. Looks like this (except aluminum-framed):

I'd like to retain the existing interior sills. What kind of replacement window frames do I get? Do I get block frames such that the interior frame is just smaller than the sill, and fill in the exterior frame with wood? Or do I get Z-bar frames?