Marc
Although I could not find the Bizzarini entered in period, it has raced in later Goodwood Revivals, so it would be legal. The Bizzarrini is a pretty cool looking car.
Randy
It can be difficult noting the difference in 289 and 427 racing Cobras, especially in slot cars, particularly during this time period, as car bodies were modified to fit wider and wider tires. Originally, the 289 Cobras did not have factory fender flares, except for the FIA Cobras, which had a bulging rear end very similar to the 427 cars. In addition, 289s were modified with makeshift flares in period. The 289s had a smaller grille opening that was flat on the bottom. The 427s had vertical oval brake openings on either side of the larger oval grille The 427s had large side pipes mounted along the rocker panels; the 289s either did not have side pipes, or they were smaller and located below the rocker panel. In a quick look at your Cobra on Saturday, it looked correct. The 289s had both a short hardtop roof or a longer, fastback roof. It was very rare to see a 427 with a removable hardtop. I did find that MRRC has correct 289s with hardtops:
Here is a Carrera 289:
I found this car on eBay, advertised as an MRRC. I do not think it actually is an MRRC. It is a model of the same car as the Carrera, but it looks like the manufacturer used a 427 body with the 289 roof.
Considering the difficulty in identifying “correct” 289s, I would be willing not to look too closely at a 289/427 Cobra. My main concern is with the potential wider rear end of a 427 car. If one is entered, a maximum rear track width may be required.