Welcome J. A. Neil:
You need a 4 X 6 and 2 basement jacks. Put the 4 X 6 across the floor joists with a basement jack at each side of the window, about a foot from the end of the joists. Raise the joists 3/8" off the wall and insert the angle iron upside down, making sure it has at least 8" of bearing on the wall on each side of the window.
Now go upstairs and see if there is too much of a hump in the floor where there used to be a sag. If there is too much of a hump; remove the angle iron and cut the 3/8" off the bottom of the joists ( a recriprocating saw works well in this case). Then replace the angle iron and let the jacks down.
Thanks for the pictures, they make it soooo much easier to answer the question. Maybe you can get a couple of after pictures too.
Happy New Year!
Glenn
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