Just thinking
Another item to check - was the sending unit in the manifold installed with a sealant -i.e., Permatex, Teflon tape or other type. The sender needs a GOOD ground so what you may be reading is from an improperly grounded sender. You don't want ANY sealant on an electrical water temp or oil pressure sending unit. If there is factory sealant, that's OK as it is a dielectric sealant. If it is a Bourdon liquid filled tube, which most Chebbies had, sealant should be OK