Tfs instructions list autolite 3924 which supposedly crosses to RC12YC or FR5. The next colder is a 3923, which supposedly crosses to RC9YC or BKR6E.
I've got a set of tfs heads on a 408 shortblock at around 10:1 and a hydraulic flat tappet at 295 adv duration running the 3924 plugs with no issues. Ymmv.
