Being that they're the same length as the original 5O2 needle, you can start them in the #3 (middle) slot. That's the usual stock setting for most needles. If you experience break-up under heavy throttle applications in the upper midrange, that will be the indicator that you need to lean your needle setting. Leaning the needle is a double edged sword, so to speak. While it does fix that stumbling in the upper midrange, it also leans the rest of the midrange circuit, making it rather flat and unresponsive. For this reason, it's best to lean your needles as little as possible, just enough to cure that stumbling, no more. This is where those shim washers come into play. If you can cure the stumbling with just a half step needle height change using a washer then that's what you want to do.
A note on shimming the needles in the '78-'79 carb set - they are spring loaded from the bottom so you want to place the shim washer above the e-clip, between it and the thick plastic spacer washer. Placing the washer beneath the e-clip will do nothing to alter the needle's height, only preload that little spring below it a bit more, which accomplishes nothing. With your clip in the stock #3 slot, adding a washer above it will shim the needle down (leaner) 1/2 step and be equivalent to a 2 1/2 setting.
A note on shimming the needles in the '78-'79 carb set - they are spring loaded from the bottom so you want to place the shim washer above the e-clip, between it and the thick plastic spacer washer. Placing the washer beneath the e-clip will do nothing to alter the needle's height, only preload that little spring below it a bit more, which accomplishes nothing. With your clip in the stock #3 slot, adding a washer above it will shim the needle down (leaner) 1/2 step and be equivalent to a 2 1/2 setting.
... I'll be swapping needles this weekend.
