Nat Buchanan certainly used the Aston Martin DB3 but he used a shell from an ACE Bristol and made his 8 Cobras in 1958. The ACE Bristol is what I believe Mr Shelby used as a basis for his AC Cobra
http://www.motorbase.com/vehicle/by-id/43/
I have some motor magazines from 1959 with some articles and pics if interest warrants
Thanks