I don't think you will find a kit with rear springs in that price range. Rear springs are pretty pricey.
It is wise to adress the steering on a 4" lift. I drove without an adjustment on my 69 K20 for about 6 months and it wasn't awful, but so much better when I added an adjustable drag link.
With the 70 open knuckle axle you're options are a raised steering arm or an adjustable drag link or crossover steering. A raised steering arm would be the cheapest.
I have bought the majority of my lift components for my trucks from
www.offroaddesign.com. They know Chevy trucks real well and are good to deal with. You can see some of the Tuff Country lifts they sell on their site. I am pleased with Tuff Country lift kits.