The most reliable method is to copy BOTH the bank and GUID, and then rename the bank file to match the car (folder) name. then open and edit the GUID file and change all references of the original car to the new one you are using them in
So as an example, the Mustang sound line would say:
{6c72a1ed-4257-492f-abe0-5c255e0c9d18} event:/cars/ford_mustang_boss_SE/backfire_ext
you want to edit the file you've copied to the Lola to read:
{6c72a1ed-4257-492f-abe0-5c255e0c9d18} event:/cars/lola_t70_rf2_ford/backfire_ext
The single most important part is not to mess up the string of code contained between the { } as this is what AC uses to identify the chunk of sound to use that is held within the bank file.
It is also critical to make backups of the original sounds before you start, especially on the stock cars.
I spent an hour or so before the race last week trying different sounds - the best by far are the sounds from the Ford 302r ( 'ford_mustang_boss_se' folder ), as far as I can tell the 428r sounds are identiacal but can rev higher as it's apparently a NASCAR engine from reading the notes. Sadly I can't get it to show in game so haven't been able to try it.
The only extra problem is that the 302r sounds much better than the Chevy sounds so I'm now looking for a big block V8 to use on those....
Hearing a Ford go by from the pits was amazing