Renovating a building from 1922

Hi everybody,
My roommate just recently bought an old school building that was built in 1922. We need to replace the single pained aluminum windows with the new vinyl windows that the previous owner purchased. The problem I am running into is there is no wood at all around the current windows. It's all masonry. The windows are only being held in by a lil bit of mortar. The existing window measures 75"x39" and the new windows measure 74"x37 1/2". What is the best way to install the new windows? I also have to replace the lentils above the windows so should I frame build a new frame for the windows to sit in? Also, would it be a good idea to putting flashing around the sill to help weatherproof? There are 8 of these windows to do in what is going to be our living room. I am waiting for my camera to charge so that I can post pictures of the existing windows and the replacement windows that the previous owner bought. Any help would be greatly appreciated. My roommate is trying to save as much money as he can on this project. I have a construction background and am currently laid off that is why I moved in. This building is a big project and in the long run both of us will greatly benefit if we get this to work. Any help would be greatly appreciated.

Here are some pics from the existing windows and the new windows that need to go in.

My knowledge on windows is limited but I am eagerly watching this thread.

I haven't found any help or material on what to do. I think as of right now, I'm going to clean up the sides and everything frame out the area where I'm going to install the new windows. The stucco on the outside has to eventually get redone also. I think that once the new stucco is finished you won't really see the framing from the exterior and from the interior we're going to be putting a brick veneer.

Good luck on this project. Hope someone expert on windows can throw out some advice...

Good luck with your renovation project. For help, you could check out companies that specialize in historical renovations like RSU Contractors. They do restorations of historical projects like antebellum homes in addition to their modern construction and renovation jobs.