Buy David Vizard's book on nitrous, read it and put a 100 HP shot on it. It will increase your torque by 200 lb/ft. @ 2626 RPM and go down to a 100 lb/ft increase @ 5252 RPM.

Have you tuned the vacuum secondary on your 600 Holley? Look at Holley part numbers 20-59 and 20-13. They are about $25 total and will let you tune the secondary side of your Holley.

But, you really should try the nitrous. Your quarter times will be about 1 full second and 10 MPH faster. It's better than sex.