c-6 crank pilot help.

I have a c-6 trans, i need a stall converter and when i order one it has 1.375 or the 1.8*** crank pilot size. What do i measure to find out? the shaft coming out of trans with the splines.. Or do i measure the shaft coming out of torque converter? Sorry for such idiotic questions, but members on here have helped me out greatly and never led me in the wrong direction lol so im heading to the knowledge well one more time.
Thanks in advance guys.

Derek
 
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