Fitting a Boss 429 in a replica Cobra will be a bit tough. The easiest car to modify for a Boss or a 427 SOHC would be an ERA or Contemporary because the steering column runs low in the chassis. A Superformance would be more difficult because the steering column runs high, right along the side of the engine. A custom steering shaft would have to be fabricated with Borgeson style u-joints. Of course custom headers would need to fabbed as well. The tops of the footwell would need to be modified but it doesn't look like that would be a major issue or reduce cockpit room because all the work would be at the top of the footwell. I'm ready to build one for somone with money burning a hole in their pocket looking for something unique
HTH,
Mike