It's not so much a science as an art, or more simply, try and see what works. Most auto performance shops will have a variety of stepped metering rods (or an get them), and can suggest two or three to try based on your driving needs and fuel octane rating. Replacing them is a very simple matter of removing one screw on the metering rod cover plate on each side of the carb, removing the rod, and replacing it with the new one. New rod springs may also be suggested to allow the rods to go to the next power 'step' earlier or later. For the timing, you want to replace the springs on the centrifugal weights inside the distributer with ones that will bring in full advance at about 2200-2500 rpm. You need to remove the distributer cap, rotor, and timing transducer plate to get at them. It's not necessary to remove the distributer from the engine. Once that's done, you merely loosen the distributer clamp (a special, inexpensive wrench is required), bring the engine up to 3000 rpm, and turn the distributer in the advance direction until you notice a slight miss; then back off a just a few degrees. A timing light will be helpful. Retighten the clamp and drive the vehicle at full throttle up to about 3500 rpm in an intermediate gear, not low, preferably up a slight hill. If, under full load you experience a miss or knock, it will be necessary to back off the timing an additional degree or two. Don't forget this step as detonation (knock) can, over time, cause damage to the pistons. Alternately, you can go to a higher octane fuel to eliminate the knock, but it will cost more to run over the long term and, if you're not planning to do a lot of mountain driving, it may be better to just accept a little less power by backing off the timing. You must decide which is better for you.
I used the term "rejetting" as a generic term when, in actuality, the Carter AFB uses metering rods, not jets, for the power curve of the engine. It does use jets to set the idle, but these are not important for performance tuning, just for starting and idling. Hope I didn't confuse you.