Its a safe bet to say that you will need 3 HDDs if you want to go the backup and restore route. From my experience I always had to format the new HDD on the PS3 then install the custom firmware again. you will need to have the ofw on a USB drive in order to begin the process since to access the xmb restore function you need to reinstall the ofw.
After install ofw you can try to restore the backup image you create of the old HDD. its a lengthy process and I can't say for certain that it will go smoothly. Better to just have everything saved on PC and just clean install ofw, install cfw and then ftp and install and file manager you are going to use.