Car makes a loud clunk when shifting into.

One of the engine mounts limits how much the engine rocks when accelerating and when shifting directions. It may have needed to be removed to replace the radiator and when reinstalled, it wasn't centered properly. That can allow the two metal parts to rub and bang against each other. It is just as likely the rubber isolator between those metal parts is deteriorated allowing that clunking to occur. That is actually pretty common.