I like Tough Country stuff (you can get it from
www.offroaddesign.com ).
It is about the softist you can get. If you don't want it that soft get their HD springs. They are still softer then many 'soft-ride' springs.
Whatever springs you buy put an extra spring clamp at the front of the driver's side to keep the spring from bending from the push-pull steering. Also if it hasen't been taken off already there is a bracket behind the rear of the front spring eye that you need to remove. Now is also a good time to get greasable bushings and some new shackles (ORD can hook you up there as well). The factory 7/16" shackle bolts are too small and can break.
You also might want to look into adding a swaybar (and some disconnects) from a 73-up blazer to make driving it on the street better if you do go with soft springs. You will need to fab up the frame bracket and swap out the u-bolt plates.
For the rear I like doing a shackle flip. ORD only makes a 4" one for 1st gens but K5's tend to sit low in the rear anyways so doing 2.5" in the front and 4" in the rear might just make it sit level. Most people when they buy springs use 2" higher in the rear then in the front. This is very important it you want to keep the hard top on.
You should not need any steering correction with 2.5" of lift in the front. If you go to 4" of lift a 2.5" pitman arm will best get rid of the bump steer.
The rear wheels sit a bit to far forward in the wheel wells from the factory. Drilling an extra hole in the spring plates 1" farther forward from the factory one and knotching the u-bolt plate will help center it. If you don't do this 33's will rub on the front of the wheel well even with 4" of lift (at least it did on mine).
After you have bought everything else get the best shocks you have $$$ for. They should work fine in the factory mounts for what you will be doing.