If you want flex (and 35-37” rubber) spring it over. That combined with long leaves will get you flex.
It is a can of worms though and requires welding. Spring over axle is the best way unless 33” is the biggest tire you will ever run.
I have built an FJ60 (very similar to a 70 suspension wise) that ran 35’s SUA and flexed like a beast but that is also a big can of worms and took a lot of work.
Now if you really want it to flex, like for the rocks, 3-link the front and triangulated 4-link the rear riding on coilivers. This is expensive and requires knowledge of suspension geometry as well as fabrication skills.
If all you want are some soft leaves in a stock height-ish application buy leaves that are light duty. No medium duty or heavy duty springs.
I am not doing a coil spring conversion for flex, I am doing it for ride quality.
Cheers