Full Frame Replacement Measuring Question - Window Ordering

I am planning on doing full frame window replacements in my home (built ‘65). The windows are original and drafty so I believe this is the best option. Because the windows are massive, mulled, and require tempered glass I would like to make sure I get this right lol. Another project required me to remove drywall so I removed it around one window to get a better sense of how they were originally constructed.

The width measurement is very straight forward, im measuring the original windows frame width which gives me a little bit of extra space in the rough opening. (53.5”, 107” total)

Where I am running into issues is around the sill. The window appears to be installed on the rough sill with blocks (instead of normal shims). The rough sill is also below the exterior brick with no flashing.

Questions: Would the height measurement still make sense to go from the top of the frame to the bottom of the window sill frame since that currently fits in the opening? Then instead of using the same block style shims should I install another piece of wood (and flash) on the original rough sill so normal sized shims would work? Finally, since I have a brick exterior would I still use nailing fins or would masonry brackets be a better choice? It looks like under the brick moulding I should have enough space for fins.

Thanks for the help!