@isiddique brother, hope you are fine.
In shadow of all gurus & expert brothers, I would say the least and dare to contribute.
The standard leaf springs on J40s are a good set that goes well with the weight of the vehicle. As the vehicle was originally designed for slow speeds off-road with load & troops' hauling, and now we need faster, comfortable ride with lesser weight in it, certain adjustments are inevitable.
Without replacing/changing any major/expensive item of your vehicle, what I think is easily doable is to
a. remove the smallest/lower leaf from all sets. Thats the load carrier. You will be amazed how much flex u'll get after removing that.
b. As a general rule of thumb, they say it should be 45 degrees. My understanding of it is that:
If leaves are flatter....the shackle angle should be more than 45 deg. with reference to ground. And
If leaves are arched properly,....the shackle angle should be between 15-45 degrees with reference to ground and not the top leaf itself.
c. With the existing set of tires & tarmac use, considering the weight of the vehicle, I suggest trying between 20 - 25 psi. There might be almost unnoticeable effect on acceleration in drag-like situation and fuel economy but the comfort will be worth that cost.
d. Checklist of leafspring service
Ensure all the bushes are replaced on every service or when damaged.
Ensure all the pins are straight (not bent in the middle) without deep groves or cuts that would cut the bushes.
There should be no gap between springs when placed in position while servicing/tuning upside down untightened. (hope I'm able to convey what I mean LOL)
Liberally use grease (not oil) between the leaves. That keeps dirt & water out for the maximum period and flex is maximized.
The pin collars should be in good shape so as not to squeeze the bushes tight.
That all for now that I can suggest.