The family background I come from, marriage to cousins is a far thing, even marriage in the same village, or last name is not allowed. I realize that caste or last name does not have any significance in Sikhi but I have seen that when it comes to getting married in the same village or last name, even many Chardi Kala Gursikhs don't favour it. They argue that it is not about caste system but getting married in the same last name does not feel right to them because getting married to their own last name would amount to getting married to one's own brother or sister.
Few years ago, there was talk about marriage into the family of very Chardi Kala wale Gursikh from Bhai Sahib's times. when he was asked whether he gave caste any importance, this Gurmukh jee very humbly submitted that caste (so called low or high caste) had no significance to him except that it would be better that if the last names of the two were not same i.e. he was a bit hesitant (not totally against it) in getting couple from same last names getting married.
On the other hand, there are people, especially from the Rawalpindi (now in Pakistan) background, who have no qualms over getting married to even their first cousins. Majority of Sikhs (including Daas) feel that getting married to cousins amounts to incest. In our culture there is no word for "cousin" because cousins are considered as siblings. When we were growing up, we saw no difference between real siblings and cousins. Daughter of your Mama ji or Chacha ji was same as your own sister. No difference.
I have seen only one statement in Rehitnamas that prohibits one to get married to persons of same last name. It is quite possible that this Rehitnama is written by a person from Jat background and this condition may not have anything to do with Gurmat. If a person of the same last name is not even a very very distant relative, then perhaps there is nothing wrong in getting married to such person but there is still strong opposition to these kinds of marriages within the Sikhi culture.
The best thing is to consult senior Gursikhs of your area about this issue and take guidance from them.
Kulbir Singh