clint - i'm currently running a 75F on my 71K - there are some slight differences in the mounts and the overall diimensions of the two tanks...
the F tanks, the Super Sport tanks, have the same front mounts as the earlier K tanks, however the tanks are longer in overall dimensions.
the rear of the F tank hangs out over the rear mounting bracket by an inch or more, whereas the rear of the K tank is flush with the rear mounting bracket.
the most noticeable result of these design differences is that the F SEATS are shorter! than the K seats...so, if you put a K tank on an F bike with an F seat you will get a 2-3 inch gap between the front edge of the seat and the back edge of your new tank.
also. the F tank's rear mounting bracket is WIDER than the K body's bracket seat... that means that your rubber tie-down cushion will not fit properly - though as I've done the OPPOSITE of what it is you are trying to do I've just stretched the rubber around the wider bracket and it seems to work just fine, if looking a little wonky (but the seat covers all this up).
junkie out.