You might find this page with valve, valve lift, and spring data of interest. Look how little load is available even when those springs were new! I wouldn't try and run those with any kind of performance cam.
Imperial FSM page
If you're going to the trouble to buy and install a cam and lifters, changing the springs isn't that much more work. Even with the heads on the engine, with the right tools it can be done pretty easily. Of course if your guides and valve seats are shot, and you're going to rebuild the heads, you've got a lot more options. Is the engine together and running?
I did a cheapy-rebuild once on a '67 383 with the stock springs and guide heights, so I used the smallest Direct Connection "RV" cam (260/.434 hyd). No interference or coil bind. It didn't make power past 5500 anyway so float wasn't a problem even with the old springs.
-Charles
ps Don are you sure you can go all the way to .550 without cutting the guides?