I am a nerd when it comes to analyzing things. On the final drive spline there are 28 teeth.  There are a corresponding 28 teeth on the wheel spline. The spline pitch diameter is really close to 2.5”.  2.5 x 3.1416 = 7.854 pitch circumference / 56 teeth = .14” per tooth. If the spline teeth were 2/3 worn, .14 x 67% = .094 tooth loss. .094 tooth loss on the 2.5” pitch diameter = 3.075 degrees of free play rotation. 3.075 degrees of free play rotation at the outer diameter of the tire (26.5” diameter for my 120/90-18 tire) = .712” free play rotation at the tread. So, your restored splines (if they were 2/3 worn) should have reduced your total drive line slack by about 3/4” - maybe now  1-1/4” in first and 3-1/4” in fifth gear measured at the tire tread.

It doesn’t seem like that would cure your problem. I wonder if your old final drive had some other issue?  Whatever, glad the on/off bucking is gone!

@4949 I can't match your level of analysis, but here is a little more on that issue. A lot of it comes down to my new mechanic, Jason Metzner of "Boxer Barn." I am very impressed with him so far! He said that I had two options, The first is to send the final drive to one of two specialists in the country that still rebuild them. He doesn't have that capability. That specialist would have either installed new ring and pinion gear or have welded on the spline portion. It would have cost over two thousand. The other option was to find a final drive with little wear. This I accomplished, and he just bolted it on. As for the wheel, he had hoped to sell me a used snowflake wheel he had on hand, but turned out he didn't have one. But he placed the used drive into my wheel, on the bench, and it was a good tight fit. He showed me that. He seemed a bit surprised himself. I think it is possible to remove the riveted spline portion of the rear wheel and rivet on that new part but he deemed further work to be unnecessary. And he is a stickler. He virtually insisted on my allowing him to fix various other problems that I wasn't aware of. For example, he replaced the rod bearings, piston rings, and timing chain. Being such a stickler on other issues, i am hoping that his judgment is correct on the splines.

By the way, here is a list of the work I have done on my bike in the past year.

1. Improved seat attachment (custom solo seat)
2. Re-shaped (enlarged) saddle bags
3. Painted saddlebags yellow
4. Revised luggage rack to carry larger objects
5. Rebuilt carburetors
6. New oil dipstick – old one had caused overfilling, leaking
7. New clutch plate and rear main seal
8. Rebuilt transmission (all new bearings)
9. Rebuilt shifter linkage
10. Various new rubber parts, like swing arm boot and shifter linkage boot
11. New throttle cables
12. Cleaned and lubed spark advance and starter motor
13. New (slightly used) final drive - this stopped the roll-on roll-off bucking
14. New rod bearings and piston rings
15. Honed cylinders
16. New timing chain and tensioner
17. New fork seals
18. Cleaned or media-blasted most bare aluminum parts
19. New rear tire
20. New rear shocks
21. Removed fairing, reverted to original headlight, turn signals, etc.

This feels very good! This is still not a complete restoration. I would like to strip her down to the frame some day. But she is very rideable now. Things that have bugged me for years are fixed. Deep mysteries have been solved. I see again that lovely bike I bought in 1982! If only we could renew our aging selves likewise!

Very good.  Good to find a competent person you can trust for help. That’s quite a list. My ‘78 came out of semi-retirement about 5 years ago, when I went into retirement.  I too have a long list of tasks accomplished, plus a few to go.  Right now, my front master cylinder is on its way home from Apple Hydraulics, and a new set of steering head bearings are waiting for me on my work bench. My wife thinks I’m a bit off with all this fixing and fussing on an old motorcycle, but honestly I really enjoy it.  It’s very therapeutic. 

You've got some great suggestions from some great guys.

ST is right. Start with the simple stuff.