I've seen what you are talking about. Most of the ones I've seen are made by a radiator shop using an inline pipe with a radiator neck welded or brazed to it. I would check with a radiator shop to see if one can be made up using the correct size pipe to fit with the size hose you have.
If you have one made up, the neck used might be from a later model vehicle and the neck might be a pressurized neck that will use a pressurised cap. You need to be sure your radiator will hold the pressure you are expecting it to hold. A 79 Chevrolet radiator will accept quite a bit of pressure, but I would be sure it is in good condition.
Joe
