in Terry Gander's book on the SdKfz 250, he says only a few prototypes of the SdKfz 250/8 were built on the alte body, although some saw service.
According to "Schutzenpanzer", by Bruce Culver and Uwe Feist, the alte SdKfz 250/8 didn't use the 7.5cm K51(sf) mounting. This makes sense, because (as you say) that mounting was designed for the neu version and the SdKfz 251/9 Ausf D. It would have looked awkward on the alte hull. I can't find any photos of one.
Both Gander's book and "Schutzenpanzer" say that production of the neu hull began in October 1943, but the changeover wasn't really complete until July 1944. Only a very small number of SdKfz 250/8s were built - 8 by the end of 1943 (some of these were presumably alte versions), 10 in 1944 and 51 in 1945. So the alte SdKfz 250/8 was so rare as to be insignificant. I'd suggest there are more important versions worth including as options in an alte kit.