If your ancestors lived in the Texas, Tamaulipas, Nuevo Leon, and Coahuila region and had a marriage dispensation before 1779 it had to be granted by the Bishop of Guadalajara since this huge area was under its ecclesiastic jurisdiction.
The good thing is that you do not have to browse through the thousands of records since Raul J. Guerra, Jr., Nadine M. Vasquez, and Baldomero Vela, Jr. have already done it for us. All you have to do is to get a hold of their two volumes titled;
INDEX TO THE GE INVESTIGATIONS OF THE DIOCESE OF GUADALAJARA Provinces of Coahuila, Nuevo Leon, Nuevo Santander, Texas 1751- 1779
Check availability at the University of Texas Pan American
When Crispin Rendon told me in an email that these two books had been the best investment he ever made, I was a bit skeptical until I had the opportunity to browse, search, and find my ancestors in its pages.
Just as a small tip I have noticed that these two books go hand in hand with the following website, http://www.guadalajaradispensas.com/ when trying to locate the original document.
If you live in the Rio Grande Valley you are in luck since both books are available at the University of Texas Pan American to check availability check the above mentioned links.