Speedway 350 tires

Hi All;
I am trying to select the correct Speedway 350 tire for my 1967 Shelby
(build date 1/12/67).
Kelsey tire states that only the first 7%-9% of the total production
received the large letter tires and the remainder of the production run
received the small letter tires. I contacted the tech line at Kelsey tire
and they could only tell me that is what their records indicate and they
could not provide any further information (documentation as to date
last large letter tire used, etc.)
I was informed that documentation does indeed exist on another forum,
but I am unable to locate it.
Is there anyone out there who can help me make the correct selection?
Thanks in advance for any and all help.
Detailguy