If I remember correctly you should not take all of the air out of your airbags, it will ruin them. I think there is a minimum pressure to avoid damage. Follow the old diagnostic rule of only changing one thing at a time. Fix the bushings first, and re-evaluate. With the right wheel and offset you should be able to fit a 10 inch wide tire inside the wheelwell without a problem. You can also box the trailing arms to reduce twist if that appears to be a problem after the replacement bushings.

To lower the front I suggest dropped spindles rather than the 6 cylinder springs. The 6 springs work great on the dragstrip but will definitely adversely affect handling on the street. There are a lot of customs suspensions available for the Chevelle/El Camino platform is you search around. It depends on what you want to achieve, track only performance or all around handling.

Nice looking truck! Good luck with it.

Pat