Just to clarify a few things from my posts above: my truck was around 5,500-5,800 lbs pre-Cummins, now it's about 7,200-7,500 or so with diesel, NV4500, heavier rear axle etc. I originally purchased brand new Eaton Detroit Springs for the front, which they rated as 250 lbs overload per side, so 500lbs overload factor. They cost me a chunk, especially shipping. The 360 I pulled out weighed about 700 and trans 100, Cummins about 900 and trans 200, so I should have been fine.
Within 3 months the brand new Eaton Detroit Springs were flat and by 5 months the front spring shackles were resting against the frame. Eaton wouldn't work with me and would only allow that if I shipped them back they'd 'Inspect' them. If they decided there was a problem they'd credit me, if not, my problem, plus I'd have to pay shipping back to them and again on any replacement springs, so 3 times the original shipping. They cost of the shipping to do all that was more than a set of springs.
I ended up yanking them except for first leaf with eyelets, got the rear spring leaves off a '49 Chevy 3/4 ton and installed them with the primary leaf. Far better and still hanging on a few years later in 2013!
RE: mileage, I've gotten 32mpg out of mine on longer trips, much less in the mountains on short trips. Running 4.10 gears and 235/75-16's. Changed tire size to about 2" taller, wider & softer with traction tread and super mileage went to hell!
It's all about gears, Gears, GEARS!