John - I did the same thing on my 46. It works fine, just don"t go over a 7 lb cap. I was told by my radiator man that the seams in the old radiators won't take any more than that. If you are worried about it you can go to a 4lb cap.
Brian & Phil I think you misunderstood what he has done. there is a overflow/vent built into the modern filler neck. It just dosen't allow the low pressure to push the water out like it does with the old non-pressurized system. Once the pressure reaches the rating in the cap it is released, just like on a modern truck. You can even install an overflow tank and any fulid pushed out will be recycled.
John, glad you got your problem whipped
George
