The seat difference between a C seat on a 777 versus a 767 C seat is that the 767's C seat is an inch or inch and one-half smaller in width. In F, the 777 has sleeper suites, but the 767 has only a few aircraft that have been fitted with the new F sleeper suites, so it is unlikely you will have a 767 sleeper suite, and it is not worth the money as the retrofitted aircraft are flown on international routes.
If I were in your position, I would simply buy a coach ticket and then upgrade from Y to C on three class aircraft with e500 Mile upgrades as this is the most efficient use of them. If you are set on buying a C class ticket, then have your seats downgraded and then request an upgrade with e500 mile upgrades from C to F. If you do not have any in your account, Premier and higher flyers may purchase a set of 4 e500 mile upgrades for $250, or non Premiers may buy 4 e500Mile upgrades for $400.