Strategy

Group Help on Methodology Required,
I need to know the correct methodology because for the first time I'm researching surnames that are plentiful:  Sepulveda and Morales.  And to complicate things, these surnames intermarried alot in the same time periods and locations. 
 
From film research, this is the last thing I am certain of: 
Maria Luisa Sepulbeda married Juan Fermin Santelices
02/27/1793 in Dolores Hidalgo, GTO
Her parents are named: Gregorio Sepulbeda and Manuela Morales.
 
I can't find either definite baptismal entries, so I go their marriage.
 
I found a Joseph Sepulbeda married Ma Manuela Morales 12/07/1774 in Snta Barbara, Chichuachua.  
 
A Joseph Sepulbeda married Ma Barbara Morales 07/08/1777
in Chichuachua, Chichuachua.
 
A Valentin Sepulbeda married Ma Eulogia Morales 10/04/1756
in Cuidad Victoria, Tamauliapas.
 
My immediate reaction was to go with the first couple because it's the best match, but am I correct in assuming this?  It didn't specifically mention Gregorio, just Joseph Sepulbeda, but knowing how interchangeable they were with their names, on the other hand, it did have Ma Manuela Morales. 
 
When is enough evidence, enough.  Or how would you suggest finding more to support my assumption.  Your help in this matter would be greatly appreciated.   Thanks so much,   Alice