TOPIC:

Wondering if I can swap out the rear fairing on my 82 kz750e with the duck bill style one on a 77 or earlier or is it to different

Direct swap, probably not. see eebbaayy photos for comparison.

Curious on this too... Did you conclude anything Jon8329 ?
Looking at ebay frames, it seems that the rearmost portion of the tail is a bit rounder on the earlier bikes.

Certainly someone must have already tried this, no?

I'm guessing an earlier seat would need to be used to look "right".

If it helps, I've just fitted a 80 750/1000 ducktail onto an 82 gpz frame and I have to fabricate every mount, and even change the seat lock, also cutting some off the rear of the frame and making a new rear section, It was quite a big job.... Not to sure with the older 900 style but I doubt it will go on without some modification..  Its now the Black bike in the 2nd picture

 

It was a lot of fiddly work..

The 750E seat overlaps the rear fairing by quite a bit and then the seat angles down towards the front of the bike.  It's a unique application and as 750R1 mentions, very different from the tail and seat on other bikes.  

Yah, thanks... upon closer look at the frames the area under the seat also looks different on the KZ750 frame relative to the KZ650.  I found an unconfirmed statement saying that the frame rails under the seat were made 20mm lower on the 750.

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The 750E seat overlaps the rear fairing by quite a bit and then the seat angles down towards the front of the bike.  It's a unique application and as 750R1 mentions, very different from the tail and seat on other bikes.  

Exactly, I bought a brand new seat to match the tail piece and ended up selling it, It was quite a bit longer, I've ended up modifying the 750R1 seat base to suit.... {even more work}