in the 70's most c&c's had windows with frames,im not sure about the landfall although in 78 most of their boats had changed into the modern look.
if they have frames they will require new lexan,butyl tape and the replacement seals.easier assembled off the boat and then sealed with thin butyl when fastened back on.
if they are the lexan type that are bonded to the deck than you can use the sikiflex.
the thing youll need to figure out is how to press them down long enough for the silicone to cure or the windows will not sit flush and start to leak.
on my freinds 30 footer we used suction cups on either side with a piece of spring steel that kept tension on the lexan until the silicone dried.we used an automotive grade urethane to bond them(cant remember the name of the stuff we used,but it worked real good and 2 years later they still dont leak).
c&c 33 tall rig