A worn clutch will not cause gears to slip out. They are almost independent systems.
A bike will slip out of gear for these reasons:
1) Worn dogs (TwoTired gave a good explanation, but there can be much more than just the dogs)
2) Bur on shift drum labyrinth (can occur)
3) Slop in the play of the large detent roller (this happens naturally with usage…look for slop around the rivet).
4) Chewed up/ bent forks (this is usually a symptom and not a cause, but can be caused with abuse)
5) Shifting paw bent/ cracked (this is caused by an #$%* previous owner trying to stomp on the shifter because they think it can engage with more force. This is one of the worst abuses of a motorcycle that can possible occur as this will screw up most of the parts that come after it, especially wrecking havoc on the forks)
6) Drum "star wheel" grooving (this happens naturally with usage.)
All of those parts have to work together 100% flawlessly. ANY error with the tolerance within that system will cause the motorcycle to slip. When you inspect transmission parts, you have to pay the utmost attention to all things. ALL "questionable" items MUST be replaced. All things should be properly measured and miked. Why? Because it sucks to have to go back to the transmission. Doing it now with the help of Bill Lane with my CB450 and all transmissions are extremely similar and work the same way, so this should apply to the CB750 too.
Notice clutch was not listed. To test to see how your clutch is working properly, ride your bike at a very low speed and shift into a gear that is too high and give it WOT. If it doesn't slip, then it's not the clutch.
Follow the adjustment procedure in the manual to adjust your clutch pressure screw while you're at it. Change your oil if you haven't in awhile with approved oil. It doesn't hurt to try these "easy but most likely not it" things before you have to spend 1-2 months fixing the transmission.
Sorry if it's not what you wanted to hear. Assembling a transmission will only give you so much knowledge (sorry, not much). When you have to tear back into it to fix problems, spend time researching, reading, and learning from the experts of all the possible problems and fixes, then it quickly becomes a rewarding experience. Then, imagine tearing it down YET again because you didn't fix the problem. Time to learn more (which will be all there is to know) and that's when you will nearly master it.
Good luck
