The John Deere 9500 was introduce in the late '80s... '89 if I remember right. The closest models available for the era you are looking for would be the ones that are included in the DPM "Olson Feeds & Larsen's implement" Kit (#661) Now sold by Woodland Scenics.
There are two included in the kit and they actually represent a Case combine but at the time most combines looked quite similar. The kit was quite expensive (I think I paid $65 for mine off ebay) and unless your a nut like me you won't want to spend that kind of money for a couple soft metal combines. There is a guy on e-bay that is selling, what looks like, re-cast versions of the machine but the castings look very crude as does the paint (just search "combine" in the N scale category and wade through the endless passenger cars that come up to find them). The DPM combine could be modified into a decent model 55 with some work but the cab would need to be centered on the body.
Model 55
You may be able to do a half decent model of a 6600 JD combine using the 9500 from GHQ but it would involve moving the cab from the center to the side and cutting away alot of the metal body (the 6600 was quite a bit smaller than the 9500).
9500
6600
The 6600 was first introduced in the very late '60s... '69 or '71 maybe even '72 so it might be a little new for what you need.
Athearn has a model 7700 but it is in Horribly Oversized scale (HO) so they may come out with an N scale version in the future which is what I am waiting for.
I hope that helps!
Mike Maisonneuve
Modeling the Northern Alberta Railways' Peace River sibdivision in N scale
http://nscalenar.blogspot.ca/
