I had what I believe is a similar if not the same tank replaced at Source Enginering in coberg. The cost was $450 but that was installed price when I used them to replace the radiator a year and a half ago.
They claimed it was designed by them in aluminum for The poly? tank that was used in Monacos (their specialty) and many other big engine coaches of that era. True or not I don't know. Pretty simple tank I suppose any shop working in aluminum could make.