I would use Carbon Copy Cloner to the FW drive formatted HFS+. Then test and verify you can boot from it. You should be able to without a problem. Then set internal as start-up disk, disconnect FW drive, reboot and install Panther on the Pismo.
OT : what drive did you put in that? I have a Pismo too and want to replace the original drive that has an obnoxious whine. It sure does run Panther a whole lot better than Jag.