Steve:
You need to have the overflow sealed up and install a new filler neck with the overflow right on the neck. Which is just like most of the modern filler necks. Then you can run up to a 7lb cap. No more than 7lbs though, the radiator seams won't hold more than that.
I"m running a 7lb with no problems. Just be sure your radiator is in good shape.
George
