toyota rears in front mixed with izuzu trooper springs. then for the rear get 63" chevies and take out the over load and other leaves if you need to. make sure the spring pack has 3 leaves. that should be soft enough to get good flex out of them without much weight in back. ive heard good stuff about mazdas in the rear, then i heard they break.. 
I think it really depends on the condition of the springs. On my 3rd gen long bed, my 3 leaf chevy springs blow. My truck is pretty light in the rear after the 13" bob job. They are really stiff and don't nearly flex like my rears up front do. The springs came off of a low rider that never hauled anything (bed had zero dents or scratches). And yes, my shackle angle, etc. is correct.
A buddy of mine with a full bodied runner runs the same spring and they are super soft. Another buddy of mine has them on his p/u and said his are really stiff. I've heard of others though that say they are really soft with their light weight rigs. If I had to do it over, I would try and run some mazda's or something a little shorter that were a little softer.
