It will depend on the torque of the engine as to which will work for you, a small block will probably want an 8", a big block a 9". Be wary of ATI, while I am sure they make good stuff their phone techs are seriously lacking. I called a few weeks back to order one, after giving him my info (447 SB2 PTq@6400, PHP @7500) and suggested to him I needed 6600 to 6800 stall, he said the one he had picked would be 6000. I asked why would I want the stall so far below my peak torque, he had no answer and said he would have Charlie get back to me. I called back twice and still have received no calls....