Sodium melts at just shy of 100 degC, and it's quite reactive.
You might look into instrumentation for sanitary applications, stainless steel, NEMA 4X. For safety reasons, I would imagine that threaded fittings would be a bad thing. You wouldn't want traces of solidified sodium around when pipes are disassembled.
A tri-clover (sanitary) fittings may fit the bill, however, you would need to perform experiments of the reactivity of various gaskets. I've had good luck with Viton in mild reactive solutions. I can't imagine any gasket material that liquid sodium wouldn't react with.
For curiosities sake, why would sodium be used for heat transfer, and in what application????