There is no need to package your game if you’re selling it on Steam.
You can just build the install folder by copying over the files from your UDK install to a new directory. Be sure not to copy over any of your code or raw asset files from your Content folder.
If you install your game from the packaged .exe generated from Frontend, you can use this to see which files you need to copy.
And for the 64 bit, you just need to copy the Win64 folder and launch the game from the UDK.exe in that folder.
You should create a .bat file to copy these files automatically so you can generate a new release build whenever you want.