You can find the Adam Strange LCs, that correspond to volume 2 of his archive, right over here:

p206.ezboard.com/Adam-Str...2881.topic

Several more DC letter column threads can be found in The Vault, just look for the magic catchphrase "what they were saying then":

p206.ezboard.com/fmarvelm...ansitefrm6

Gormuu has a database of old Marvel LCs too, and that can be found over here:

marvelmasterworks.com/res...month.html