In the Metro terminology this is called a remote build (a stage 3 of a different, but binary compatible, architecture is used as a seed). What's not compatible? You can't use a Sparc architecture to generate an x86 or ARM based stage and vice-versa. If you use a 32bit stage then you don't want to seed a 64bit build from it. Be sure that you are using a stage from the same architecture that you are trying to seed. Check Funtoo-current FTP Mirror for a stage that is from the same Architecture that you will be building.