One of the big differences is the aluminum body for the Kirkham, vs fiberglass for the Superformance.
But there are many differences. I think the Kirkham car is a much more "correct" car, like the originals. Chassis, and battery location, etc...
If you want the original look of a 427SC, and then of course a big block FE motor, then I would go Kirkham. (Money not withstanding
).
If the aluminum body and "more original" look are not important than buy the Superformance.
I assume you are only looking at used cars right?
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