265-350 SB-Chevy Internal Balance 6-1/4'' Diameter
400 Chevy External Balance 7-1/4'' Diameter look on the back side for the counterweight.

Fluiddamper does offer a 7-1/4'' 265-350 SB-Chevy Internal Balance. so it becomes important to look at the back side in order to verify the part. Diameter is a quick way to tell, counter weight confirms the part.