I emailed Kevin at Mart Auto Works and he said that I should try posting this question on a forum. Which did the cars come from the Factory with? The Tachman said it would be a 6k but he wasn’t 100% sure.
My first Mustang was a 68 "S" code FB with factory four-speed. It had the 8K tach.
The 67 "S" code FB four-speed that I own now came from the factory with a 6K tach.