I would put a stock airbox on it with a K&N drop in filter. Jet the carbs for what they need, rather than a "stage" of "jet kit." The airbox will make the bike run better overall (especially at cruising speed around town) and it's easy as pie to install.
For the rear disc setup, you could get a 1975-6 CB750F (super sport) swingarm, rear wheel (spoke wheel with disc), and caliper, but you'll have to figure out how to mount a rear master cylinder and brake lever, because the frames are different between the f and k bikes in that area. (k is setup with a welded brake pivot on the frame for the rear drum, the early f has a bolt on bracket for the brake pivot and master cylinder).
How good is the stock paint? If it's nice, you would be smart to sell the tank (and side covers) you have and find a tank that has either has been repainted or has damaged paint and paint it to suit. Check out ebay and see how much nice tanks go for.

You'll have a fun bike when it's all done. Good luck!