You need to cut the mounts for the bottom and reweld them to where they should be then find a hole on the upper support which is threaded for your rad stay. Do you have ac still? If not you need an ac block off plate off an eg or your rad won't work.
Thinner and longer radiators have the best cooling efficiency. Unless if space restriction prevents you from using a full size radiator, I don't see any reason to use a half size.
My half core is in without the civic block off and my temps haven't gone up. 3 hour drive on the highway and driving around the city for an hour confirms this. The civic block off wouldn't be a bad idea though.
it isnt cooling as efficiently as possible. the radiator cools coolant from air passing through it. if you have no ac and a half size radiator air will take the path of least resistance and not go through the radiator.