904 Flexplate??
Call Summit or Jegs.....get a converter for a 904 trans and a flex plate for the converter.....should be a 6 bolt crank flange. Order the P/N that drjay0 posted and you will be just fine. If you have a 318 the flex plate will be different...you will need a flex plate for a neutral balance motor. 318's are internally ballanced. If want even less confusion call Mancini Racing or Cope Racing Transmission and tell them what you are driving, waht the car is used for and they will get you what you need.