If you are going Concours, depending on which division you choose, Radials may get point deductions.
As far as Coker versus Diamond Back, I have never used Diamond Back brand, but have been using Coker tires for over 10 years and have never had a problem with them.
I just recently bought a new set for my '67, and found Summit and Jegs cheaper for the Coker tires. Summit came out a bit cheaper since their shipping was less than Jegs.
Bottom line, Summit was $35.00 a tire cheaper with shipping then buying the same tires directly from Coker.