I actually use the Barrett Jackson cars quite often in the online Hoppers.
Ford Roadster is a beast in C and B class
Plymouth Bubble Top is a very nice acceleration car in B class
The Chevelle can be tuned to run pretty good in B Class
The Bronco is a beast acceleration car in C class
The Cuda is good in C class and okay in B class
As stated before, just because you guys can't put your "cookie cutter" S2000 or 97 Civic builds and tunes on cars, doesn't mean they are not good cars. Anybody can win races in a S2000 or 97 Civic. Build and tune a Ford Roadster and win races in online Hoppers.
It's a very bold assumption to make, that everyone who doesn't like the car pack must be the type of people who only use cars like the S2000 or Civic. Surely it can't be because they don't like the clipping issues on the bubbletop of the Bubbletop, or the ugly rims on the Chevelle, or the lack of a roof-down option on the 'convertible' Cuda, or the fact that the Bronco is yet another SUV/offroader when there are more SUVs and offroaders than there are cars in the Forza P1, Forza GT, and Forza Touring Car divisions, or the necessity of putting ugly Forza aero on these otherwise (mostly) beautiful cars if you do want to race them, or the aforementioned inability to race these hotrods/customs against the vanilla hotrods/customs unless you specifically set up a custom ruleset for it, etc. etc.
I'm glad you like the cars, but it's entirely reasonable that people, myself included, dislike it for reasons outside of 'oh we didn't get the 2003 S2000 like Horizon did so it sucks' (though that being said, there's a lot I could talk about comparing the car passes for each game).