I'm sorry my terminology isn't correct, as every so how window I have looked at that installs the way I'm talking is called a "replacement window" on every box stores website.
Tcianci:
Is there usually enough to take off 1/4 To 1/2" with a plane?
Also I found this window...
https://m.lowes.com/pd/ThermaStar-b...n-x-36-75-in-Actual-27-5-in-x-36-5-in/3119105
It's short... now I've never installed a window but I remember an old buddy installing one once, and his was too short but came with an extension that went on the top, like a cap, to fill the space... do windows usually come with these? Because I would only have to make up about 3/8-1/2 inch to get a good deal against the top stop
The window in your link sounds like it would fit your opening. "Short" is a relative term. The window installs against a feature of the old window called the blind stop. If your new window is tall enough to overlap onto the blind stop of the head jamb, you're in good shape. If it doesn't quite make it, then you would use the head expander.
Replacement (insert) windows are usually supplied with 2 accessories, the terminology of either may vary by region. The 2 accessories are the sill extender and the head expander. The sill extender is just that, a piece plastic that fits onto the bottom outside rail of the window to allow the window to fit along the sloped sill of the old unit. When installed, the new window is essentially resting on the inner bottom rail and the bottom edge of the sill extender. Sill extenders are nominally sized for window sill with a slope of 14 or 15 degrees. Some manufacturers, like Andersen, actually ask for the sill slope when you order windows from them.
Some sill extenders are simply a vertical piece of plastic that snaps into a groove in the face of the bottom rail of the window and these extenders can be cut for a sill of shallow slope. Other sill extenders have an additional feature at the bottom that gives the extender more contact area against the sill and helps the extender to stay rigid. They too can be cut but the rigidity of the extender could be compromised.
The other accessory is the head expander, It is a vinyl channel that slips over the top rail of the window and it takes up any extra space between the top rail of the window and the head jamb of the old window. We typically insulate the voids at the sill extender and the head expander to optimize the efficiency of the window.
It sounds like in your case, you would be installing the window without the head expander since height is an issue. As I previously mentioned, the top edges of many replacement windows are raised above the main body of the rail itself and this feature can be trimmed off. It is however, unlikely that there's more than 1/4 of an inch to be had in this area. If you were to trim this feature at the inner and outer edges of the top rail of the window and the same feature at the inner edge of the lower rail and a corresponding amount from the bottom edge of the sill extender, it may be possible to "shrink" the window by about 3/8".
At this point you may understand why several have advised you to spring for the custom sized window. However, if your handy and adventurous, you may save yourself a few hundred bucks.