saddle position is not a good way to adjust the cockpit space on any bike. the saddle position should center balance your weight on the bike and create an efficient pedaling position. saddle position is 100% about how your pelvis is located in relation to the pedals. if you can't achieve a balanced, efficient pedaling position from the saddle with the existing seatpost, you can move it forward and back a bit, or use a seatpost with a different offset. if you need to push the saddle all the way back on the rails, you're likely to bent those rails. the saddle will be most stable and strong close to the center of the rails.

positioning your upper body has nothing to do with saddle position on the bike. once you have the saddle in a balanced, stable, efficient position, place the grips where they need to go by means of stem length/ height/ and angle, handlebar dimensions (width, angles, rise), and spacer stack under the stem. if you have to use a stem length, spacer stack, of handlebar with dimensions that you find compromises handling or aesthetics, you need a different frame.

This guy's videos are a great place to start: https://www.youtube.com/channel/UCr4...yq_QM00L1xJb4A