OEM size was 11.75 (usually 440s )or 10.75 (318-383 ) and the Hemi was a high stall (10.75? ) The smaller flex plat is for the 10.75 and the big is for the 11.75. The 10.75 converter had many different stall speeds depending on the application and year. A nice cheap combo for a 440 car back in the day was to swap to a small block (10.75) converter for more stall.