hi derek,
i wont rely on fireclay etc.
instead i make the holes for the steam pipe and exhaust/blastpipe a close fit, thread same all the way down and fit curved washers and threaded nuts to cover any gaps. there is then no possibility of sealant or fireclay to really as a sealant. instead a mechanical fixing being preferred.
it is always possible to add an extra plate on the bottom of the smokebox to achieve the above.
cheers,
julian