Hey Yogi, I have a 78 CB750K with a 4-1 "F" model header off a 78 Super sport. I am using a Cone Engineering Quiet Core (don't let the name fool you, thing is AWESOME) Stainless Steel Megaphone muffler. At idle is has a very nice deep low rumble and below 4k it sounds very nice, When you really get on it, this thing screams and has one of the best (if not, the best) sound I've heard on one of these bikes. I really like it. When I was searching for a muffler flybox (member here) recommended it and I couldn't be happier. Very good quality. Just make sure you measure the O.D. on your header so you get the closest size. I had to get/make a spacer to adapt mine to the F header.
Good Luck, Let me know if you have any questions.