hahahahahahaha
Fanus... even though i can see 50 crates of quarts beers in the back of your patrol in that picture,... your springs are not coping. never seen a spring under axle pack invert like that, ever.
Look, to be perfectly honest, a flat spring pack is better for flex, and more comfortable day to day, ... but on a vehicle like a Patrol, which is Spring under axle (designed for loading), it means your ground clearance would be terrible, as well as your weight carrying reduced..... Further to this, it makes for terrible driving/twitchy steering when loaded/towing.
Think that bakkie of yours needs a new spring pack.
My advice though, don't get 400/500kg constant load. Rather get the standard pack. With nothing more than a hunting frame on the back, the uprated packs will feel like the axle is welded directly to the chassis, trust me.
I still have my original spring pack somewhere in my workshop. its done about 70 000 km. i've been keeping it to do a custom pack, mixing certaibn leafs with my XGS TJM leafs, as i dont like my spring packs at all, i feel i can do better.
I wish i could find out if Land Cruiser 60 series leaf pack would fit, as they are longer, and way more options avaible. 79 series will not, as far as i'm aware.