Here's the car description of my rental receipts from the previous V8 muscle cars I've rented:

Vehicle Group Rented: Cool Cars
Vehicle Description: RED FORD MUGT 2DR

Vehicle Group Rented: Cool Cars
Vehicle Description: BLK CAMARO SS

Vehicle Group Rented: Cool Cars
Vehicle Description: BLK CHALLENGER RT

In the Avis locations, I've noticed only about 15-20% of the "cool cars" muscle cars are the V8 versions. On the plus side, there doesn't seem to be a price difference between the V6 and V8 versions of the car, unlike Hertz which charges you 2-3X more for the V8 versions.

At Avis you can potentially rent them for much cheaper but it's a crap shoot what trim/model line you get for sure.