Doors and Windows - how do i finish the inside of an egress window

I just installed an egress window. The wall is 12" thick, and the window is for only like 4" deep inside. and there is a 1" to 1.5" gap between the rough in frame and the window. So heres the question I need to build up 1" to 1.5" around the window and make it 8" deep, Also Iam framing in the basement w/ 2x4 walls . so how do trim it out? thanks for all or any input.

Here's what I'd do:

Frame your basement walls up, leaving a rough opening that's basically the same size as the masonry opening for your window. Once the drywall is on and the walls are painted, that would be the time to install your trim. Make a jamb out of 3/4 veneer plywood (cabinet grade is usually best) that is the right width to reach all the way from the window to the surface of the drywall on your new wall. You'll nail that jamb together in the shape of a box, stick it up against the window, put shims around the outside to hold it in place centered on the window, make sure it is square when measured diagonally, and if it is, secure it to the framing with finish nails. If you want, you can run a fine bead of silicone around the jamb where it meets the window- just don't get any on the wood that will be stained or painted!!! Then put a piece of baseshoe or similar moulding around the perimeter of the window seam where your jamb and window meet. The 3/4" plywood together with a 1/2 x 3/4" baseshoe would provide the 1 1/4" that it sounds like you need.

If you use 3/4" plywood for your jamb, it's always nice if you put iron-on banding, or glue a solid wood edge onto the edge that will be seen- the edge of the jamb shows where it meets the casing because usually you have a 3/16" reveal, or step.

Thanks for the advice I will hopefully be finished this weekend. Once again thanks for the help. Kelly