Hi bb, you're not missing anything as they are different cars with different engines. They're both left hand drive so would presumably be US or Euro cars.
Conversions like these, to my eyes, look absolutely daunting and I would question the installation of the 6.3 engine in what is essentially a car with a 'soft' chassis. Even in a 109 body, the weight of the engine can take its toll on the cross member which can crack or become out of alignment. Thus, the longevity factor certainly needs to be taken into consideration.
I recall another thread in Topklasse which featured the German 450SLC fitted with the 6.9 engine and that RonB in Brisbane had a hand in. The manhours, AND money that went into that project were quite staggering. I sometimes wonder what fate befalls these complex conversions - you know, still revered and pampered today, sitting in the corner of some garage or shed...who knows. Regards Styria