68' CL450 Issue, 5th Gear Slipping, Possible Clutch Issue?

Two years ago my friend and I rebuilt my 70’ CB450 engine with the bottom end of 68’ CL450. It was riding pretty well for about a year (for the exception of some power loss). But about a year ago I went for a ride and while riding in 5th on the highway, the whole bike jerked like the clutch slipped and caught several times. So I backed off and slowed down. I dropped into fourth and everything was fine. So I shifted back up into 5th and the same thing happened. I did not hear any knocking or metal crunching around in there.

Testing:

• My friend, who is a mechanic, sent a camera into the engine and found no abnormal wear on the gears or the dogs.
• I am still able to ride in 1st through 4th but 5th gear keeps slipping. Again, no knocking or strange noises.
• I am currently working through all the options of what it could be. With the hope that it is not something deep in the gearbox like bent shifting forks or worn dogs.
• I recently put the bike on the center stand. Started the bike and cycled through the gears. Every gear engages and the rear wheel will spin.
• 5th gear does engage and spins when I hit the throttle.
• It appears that when 5th gear is under load on the road it slips (again no knocking or strange noises).

I have made some repairs already.

Repairs:

• I hoped the problem was the Gearshift drum center (which turned out to be fine) and the shifting pawl which was a little worse for wear.
• So I replaced the detent roller, the whole arm, and gave it a new spring. Definitely shifts better now but same issue in 5th gear.

Theory:

• Could this be a clutch issue?
• The clutch plates and springs are all original and should probably be replaced regardless
• Would a clutch adjustment also help the situation?
• I really hope this is not an issue that would require me to split the case

Any Ideas or recommendations about the situation would be greatly appreciated!

Best,
Kevin