How do I determine what date code to request for a replacement windshield I want to order for my 67 Chevelle? Do I go by the build date on cowl tag?

That's what I'd use. I don't think Fisher would start building a body without window glass. Glass is only dated by month anyway AFAIK with a two letter code; 1st letter is the month and 2nd letter is the year - A is 1966 and Z is 1967. Example "LZ" would be March, 1967.

N - January
X - February
L - March
G - April
J - May
I - June
U - July
T - August
A - September
Y - October
C - November
V - December