Poncho 62 was right with the coil springs on GMs, they will rust at the bottom when theres mud and stuff in the seat. NEVER heat your springs to lower them as you will cause them to lose their proper temper and will just usually bunch up a couple of the coils; then when you least expect it they can jump out of their seats and jam causing one control arm to lift up and thats something you really dont want to have happen to you as one of my students did exactly that when I was teaching back in the nineties.
Talk to the manufacturer of replacement springs and you will find that altering the rate by a few pounds wont matter but if you change the rate of the front pair too much without the rear also being altered to compensate you will end up with a plow understeer situation where in an emergency your front end will slide straight ahead instead of steering. So if I was in your shoes I would swap both front and rear for reset or new springs, a liitle lower in height and a few pounds more rate will get you walking around a stocker. Dont go mad on your shocks either a premium gas high performance STREET shock will do wonders, and finally yes renew all your bushes with urethane and youll feel like you are driving a different car,
Cheers from Aus