Flexplate or converter? What stall?

There are a lot of "if's" in this response. If this is for the street only and if the 904 is built to handle the horsepower, and it has 500 HP or less, a B&M 360 flex plate and the existing converter will work fine. Over 500 HP and/or the car is going to be drag raced a bit, I'd get about a 3500 stall converter and SFI flex plate. If it's going to be a full time race car, 4500 stall and a SFI flex plate to go with it.