If you want to keep the factory springs and have your truck level with any legal load, you should get TLC™ Air Suspension.
I may be biased on this……
…..but I have talked to people who got heavier springs and then still got TLC…..I don't believe any coil spring can carry a heavy load and provide a smooth ride.
Yes, TLC is more expensive than plastic sleeves that go inside the coils, but TLC is rated at 5,000+ lbs v. 1,000 for those "other guys". And with TLC you keep the coil springs (some systems replace the coils completely and if those air bags blow out, you're probably stuck on the side of the road).
Lots of info including great install pics and video from Z members in our Vendor Forum. Happy to answer any questions.