I thought that i would chime in on this, attached is the water tube that Egge Machine has for the 1937 GMC/Olds 230 engine, part # WT5. as you can see it is longer than the one that phillpj posted. The GMC part manual has this as part # 408108, whereas the 1936 part # is 404158.
Egge lists this at $77.50 plus shipping and It is galvanized steel. I don't know if it could be modified to fit.
I have reached out to other GMC truck owners to see if they have one or a source.