In the "Subtlety Optional" assassination, what counts as a muscle car?

There's a very short list - I think maybe 4 total:

• Bootlegger
• Hammerhead
• Phoenix
• Hammer (this one definitely works)

They're tricky to spot, and they're not sports cars (so the Torch/Temptress/etc don't count).

I found they tend to spawn outside the Angel's Gym crib the most often, in the parking lot and surrounding lots.

I believe the Bootlegger is also a possible gang vehicle option, if you select it from the gang customization menu at one of your hideouts. However, you'll have to figure out a way to make your homies show up if you go this route. There's an upgrade option in the phone which gives you the ability to call some gang members, which is the most reliable way.

I think the Phoenix is driven by a particular homie (possible spoiler if you've not advanced the plot to the point where you meet the Mayor):

Mayor Reynolds - he drives a Phoenix customized to look like the Trans Am he drives in Smokey and the Bandit