I agree, the post 81 clutch is much better and performance is improved. I did it to my 75 R90 using an 83 tranny.
Another reason an early, long nosed input, tranny doesn't work well with the later clutch, is the travel for the clutch activating rod is too short. The later trannies have a much different lever and rod configuration.
And don't forget that you need a 9 tooth starter.