Ford 272 C.I. runs low 11's. Why do Mopar guys need 408's?

Tomorrow i'm helping a new customer who has a seventy fast back mustang and a four sixty on an engine stand that's been dyno'ed... It's painted but it's also gutted. It has a four linked ford nine and a mustang two front suspension.. My preference is this and starting from scratch and not fixing other people's work...