Just get fw400 to fw800 cables (6pin to 9 pin) to connect the fw400 to your fw800 imac port.
And you should be able to daisy chain them.
Here's all of monoprice.com's firewire cables. 6-pin cables are for fw400 and 9-pin cables are for fw800.
If you already have a bunch of fw400 cables lying around then you can just get a fw800-fw400 adapter instead of a cable.
But the cable is cheaper.