I've always thought NO '66 HEMIs until after Thanksgiving 1965
I don't have any per month HEMI build records - I have a total number of 426 cu.in. engines for model year 1966 at 3,629 (but this would include WEDGE & HEMI engines)

a GSD Mr. Norm's 426 WEDGE Coronet 440 was recently found and discussed on For B-Bodies Only - its SPD is B22
https://www.forbbodiesonly.com/moparforum/threads/426-wedge-in-a-66-coronet.261204/

the earliest Lynch Rd. HEMI car I have anything on is SCHEDULED C10 (Friday 12/10/65) & the earliest St. Louis HEMI car I have anything on is SCHEDULED 113 (Thursday 1/13/66)

....... here's an article from 1/7/66 National Dragster