Just learned from several on line data sources that the 6.3 W109 used a larger gas tank than any of the other versions of this model...27.8 vs 21.7. Since the body shell is likely identical, I would imagine that they are interchangeable, which would not be a bad thing for the 4.5 given the lousy mileage that that US only model gets due to diminished efficiency of early smog compliance measures. Indeed, the pre 1968 6.3 got about the same mileage as the 1972-3 4.5 model!

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1959 Ford Thunderbird convertible/430
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang