A number of publications cite the need for the foam seal that closes up the gap between the radiator and the diaphragm plate on MGBs (up to 1977). Mine has never had one and I thought I was OK without it. The car has always run about a needle width past the "N" mark on the gauge and I have never had a problem with overheating. However, with the staggeringly hot conditions we've experienced along our part of the Gulf Coast, the old girl was running a bit hotter than expected.
A few MGB owners I've known extol the virtues of Moss Motors part number 282-905 "Seal, Radiator to Support" - the foam rubber packing that prevents air flow around the radiator and forces it the flow through it. They tell me that it makes a world of difference in operating temperature.
Having nothing to lose, I decided to give it a whirl and see if it would help my issue. My long suffering spouse told me that one of her favorite sewing supply stores had foam rubber sheets on hand. We bought one 2 inches thick and I cut a strip off of it 2 by 1-1/2 inches. A two foot section fit the gap just fine.
This morning, I took the 'B out for a blast to see how well the foam strip did its' job. Very nicely, it turns out. The temperature gauge was showing the usual slightly-past-the "N" running. The only down side is that the foam is light green (something to do with how wonderful it is for the environment).
I suppose I can live with that.