They're all V8's from the Hertz link:
Any Mustang with a GT badge is usually a 5l V8
The Camaro's an SS model = 6.2l V8
The Challenger R/T's a 5.7l V8 hemi
The site is a bit poor, but prices are pretty decent - Most other hire companies that rent Mustangs / Camaros use V6's
(Remember and google around for Hertz cashback deals at topcashback & Quidco / voucher codes before booking).