There is no formula for jet sizes when you switch from the stock airbox and exhaust. Even if you specified the kind of exhaust and pods you are putting on (there is a difference between EMGO and K&H pods), each bike is a little different so what works for one may not be ideal for another.
If you do a little research, you will find that your '78 K carb (assuming it is stock) came with 110 jets and in general, adding pods and a 4 into one exhaust will require an increase in 2 or 3 jet sizes, so 120 or 125 jets would probably be a good place to start. That will only address the high end rpm range however. For the mid range, you will probably have to shim your needles (the '78 carb needles are not adjustable so you will need to add a couple washers to each needle to lift them). Boring out the pilot jets may also be required to get the mixture right. Hondachopper.com has some good info regarding the necessary adjustments to the '78, including details on boring out the pilots.
If I were starting the engine after a rebuild for the first time, I would put a stock airbox and 110s on it and go through a complete tune up (idle adjust, timing, carb balance...) to set a baseline, then throw your pods on and make the needle and jet adjustments, then re-tune for idle again and go out and do a few plug chops in the mid and high rpm range to see where your mixture is at.
