I'm not sure how air assist is going to help you if you are keeping your stock rear springs. Maybe a link suspension with air bags???
Either way, you need to get as far away from stock springs if you want a decent ride. Probably some half ton springs (probably going to move rear hangers to make this happen), a shackle flip, etc. to soften the rear up along with some weight to balance out the bias. I'm quite pleased with how well mine rides for what it is. Granted it lacks greatly in the steering and handling department but the ride is better than most newer trucks I've been in. Pretty surprising for leaf springs. Its a tough thing to blend ride quality, load capacity, handling, and off road flex into a suspension without spending alot of money, some are just going to be sacrificed.