The -8 Cougar is well represented in 1/72nd scale by the Hasegawa kit and in 1/48th by Collect Air resin kit and Fonderie Miniature and not as well by Revell, whose kit was actually 1/52nd. There isn't a -6 in either scale.
The changes from a -8 to a -6 are easy to summarize: shorten the fuselage by eight scale inches and the fuselage/wing fillet as shown (aft only; the forward section remains the same relative to the mid fuselage/wing) and reduce the chord of the wing:
Note that the speed brake has to move aft with the forward fuselage so a scale eight inches has to be removed from the fuselage behind it.
Some have noted that the length of the F9F-6 is given on the interweb as 40' 10" and the F9F-8, 42' 2". That would be a difference in total length of 16" inches, not 8. However, those dimensions appear to be apples and oranges. First, what you get for length depends on whether you measure along the waterline (i.e. the top view) versus along the ground. Depending on how much the airplane sits nose up and what the difference in height of the two points you're measuring between, you'll get different numbers for the length along the waterline versus along the ground. Second, it depends on what you measure to. For example, there is the tip of the Cougar nose, the tip of the barricade strap deflector that sticks out about three inches beyond the tip of the nose, and the tip of the inflight refueling probe mounted on the nose.
Measured along the waterline from the aft tip of the horizontal tail to the tip of the barricade deflector (the number provided in the Standard Aircraft Characteristics chart), the F9F-6 is 41' 0.75" long and the F9F-8 is 41' 8.875" long. (I'm not sure where the extra 1/8" goes, but I'm sure you're not concerned about it either.)
In the case of the Cougar, the horizontal stabilizer is higher than the tip of the nose, so when its length is measured along the ground, the distance is between an upper and lower corner of that angled "box", which is longer than its width for reasonable nose-up angles.
With respect to the dimensions cited from the web, the F9F-8 measurement along the ground to the tip of the barricade deflector is 42' 1.5625", which rounds to 42" 2". I'm not sure where the -6 measurement of 40' 10" comes from. It is likely to the tip of nose when measured along the waterline; that would make the distance between the tip of the nose and the tip of barricade deflector to be 2.75 inches, which is about right. It is almost certainly not the distance along the ground; in an early SAC, that is given as 40' 11" to the tip of nose, not the barrier guard, which would be even longer. In other words, the F9F-6 length of 40' 10" appears to be along the waterline and to the tip of the nose, whereas the F9F-8 length of 42' 2" is to the tip of the barricade deflector and along the ground. Apples and oranges.
In any event, I prefer to use station drawings from Grumman to compare the fuselages, and they confirm the eight-inch difference in length.
The wing change from the -6 to the -8 affected both the leading and trailing edges (the torque box including the flaperette and flap cove location was unchanged). The -6 leading edge slat was deleted and replaced with an extended and cambered leading edge outboard of the wing fence. The chord was also increased aft by enlarging the flap and extending the trailing edge outboard of the flap.
Note that the front of the flap extends farther forward (see the dashed line in the illustration above) on the lower surface than on the upper surface. You can also see the camber in the leading edge of the -8 wing out at the wingtip.
However, the wings in your kit may vary. For example, the Hasegawa 1/72 F9F-8 is an excellent kit, but the wings are not exactly the same as the Grumman drawings:
The F9F-6 wing also folded upward until it was vertical, whereas the -8 wing only went up 80 degrees like the Panther's.
It helps to know that the F9F-6 was created by simply adding a swept wing and horizontal tail to an F9F-5 fuselage that had the fuselage/wing fillet extended forward. In the following illustration, the black lines are an F9F-5, the gray represents the F9F-6 additions, and the red lines depict the planform of the F9F-8. (The green lines show the location of the upper surface controls and flaps; note that on the lower surface of the wing, the front of the flap is located farther forward than it is on the upper surface.)
In 1/48, Don Fogal accomplished the same thing by mashing together a Monogram F9F-5 and the old Revell F9F-8. Here is a picture of the gorgeous result:
There is a possibility that the 1/48th Trumpeter -2/3, which may be more readily available than the Monogram -5, really has the fuselage length of the -5. I've added an approximate way to check that in my Panther blog entry, HERE.
For much more detail on the Cougar, buy Steven Ginter's Naval Fighters Number Sixty-Six for drawings and illustrations and Number Sixty-Nine for pictures of Cougars in every Navy and Marine squadron. They are available HERE.