If you hook up an air line into the cylender through the spark plug then you should be able to remove the spring with a spring compressor from your local parts store, then you should be able to hold up the valves with the air and change the springs that way.
That is the only way that I know how to change them while the heads are still on the block. You might be able to get the springs off another way besides an spring compressor, but I have never tryed nor do I know of a way.
Good luck!
1998 Silverado Z-71 Ext Cab. 5.7L Descreened MAF, K&N direct replacment, 32x11.5 R15 AT's,Posi rear end, True duel w/Magnaflows, Sub woofer setup, Remote starter.