Look up the thermal resistance specs and absolute maximum spec ratings in the datasheets of the parts you want to use.
Editorial comment: If you're going from THT to SMT you may want to consider redesigning the board to integrate some of the THT components if possible...lots of SMT parts have never existed in THT so you must be using old technology (yes, a bad assumption since I know nothing about what you're building - if it's high power maybe it's a really bad assumption...if it's a bunch of 4000 series gates then it's not).