It appears that your old windows were made on the spot when the house was built and now you're trying to fit a new window that obviously wasn't built on the spot to fit.
I'm probably missing something you're asking, but you will pretty much have to do some rebuilding and custom work. The new frames are made to match what amounts to industry standards while the existing framework was built to match the house as it was built. It's like trying to fit a 2017 radiator into a 1923 Ford.. Sure you can do it, but you're going to have to do some rebuilding of the old Ford..
I'd probably use thin layers of wood and build them up till things match and use waterproof glue and make sure to use a good oil based primer so water doesn't soak in and you end up having to redo it all in two years.
I figured it's time to change my signature so hold your breath. This is it.
Marty or Marty Farty if you feel mean.