According to the Shelby Registry, All the S/C cars had 427 engines. 52 Competition Cobras were made, 2 prototypes and 50 production cars.16 were sold as full-comp cars. The 34 unsold cars were a problem and a Shelby employee came up with the idea of the "Semi-competition" model. Before they were all finished that way, three more full-comp cars were ordered, so only 31 427 S/C's were made. Those cars, full-comp and S/C were serial numbers 3001-3055 and they all had 427 engines. The next 100 Cobras, CSX 3101-3200, also had 427 engines. Due to supply problems and a less costly engine( the 427 engines cost Shelby a whopping $720, vs. the 428 which cost $320!!!!), 428 engines were installed in ~CSX3201-3299, with some exceptions. The car was still advertised as a "427 Cobra" on the brochure, but it listed the engine as a 428 in the specs. See this link for the brochure.Click on "view full image" at the bottom of the page to be able to read the details of the brochure.
http://members5.clubphoto.com/_cgi-b...tem_id=3434115
The last 60 cars, CSX3300-3360 were all 427's except for CSX 3300,3301,3305,3306.