Did you ever find out if the 65 2wd core support was a better option to go with? I am in the exact same situation. I have a 66 2wd with a bad core support. I would rather replace it even if it means some mods are necessary to make the new support work.
I am restoring a 66 2wd f100 stepside. I have looked high and low and this article from bluestang65 is the best and least confusing I have found. I am doing the exact same repair with a 61-64 radiator support from LMC.
Did this turn out ok? Did you use the mounting kit for the 61-64 or 66?
Just curious if bluestang has checked out the 65 to see if it was a better support? Looking to do the front end soon and would be thrilled if I could use the 65 support and only need to swap the radiator.