I had an '81 engine in my '82 for a few years. It is not just the final drive, the gear ratios are also different. Even with an '82 final drive, the '81 engine was pushing 5000 RPM's at roughly 75 MPH. I installed an '83 engine (still in the bike) with the '82 final drive and the RPM dropped to 4200 RPM at 75 MPH (roughly). Huge difference on the highway!
However, my bike is named the, "Slug" because of this. 1st gear is a bit sluggish verses what it was like with the '81 engine. There is a trade off on either end of the RPM spectrum when you change the ratio's